Semiconductor Device, Display Device, And Electronic Appliance

ABSTRACT

In a channel protected thin film transistor in which a channel formation region is formed using an oxide semiconductor, an oxide semiconductor layer which is dehydrated or dehydrogenated by a heat treatment is used as an active layer, a crystal region including nanocrystals is included in a superficial portion in the channel formation region, and the rest portion is amorphous or is formed of a mixture of amorphousness/non-crystals and microcrystals, where an amorphous region is dotted with microcrystals. By using an oxide semiconductor layer having such a structure, a change to an n-type caused by entry of moisture or elimination of oxygen to or from the superficial portion and generation of a parasitic channel can be prevented and a contact resistance with a source and drain electrodes can be reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/897,419, filed Oct. 4, 2010, now allowed, which claims the benefit ofa foreign priority application filed in Japan as Serial No. 2009-234413on Oct. 8, 2009, both of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device, a displaydevice and an electronic appliance which use the semiconductor device.

BACKGROUND ART

In recent years, techniques to form thin film transistors (TFT) using asemiconductor thin film (with a thickness of approximately severalnanometers to several hundreds of nanometers) which is formed over asubstrate having an insulating surface have attracted attention. Thinfilm transistors are widely applied to electronic devices such as ICsand electro-optic devices and are particularly expected to be rapidlydeveloped as switching elements of image display devices. Various metaloxides are used for a variety of applications. An indium oxide is awell-known material and is used as a transparent electrode materialwhich is necessary for liquid crystal displays and the like.

Some metal oxides have semiconductor characteristics. Examples of suchmetal oxides having semiconductor characteristics include a tungstenoxide, a tin oxide, an indium oxide, and a zinc oxide. A thin filmtransistor in which a channel formation region is formed using suchmetal oxides having semiconductor characteristics is known (PatentDocuments 1 and 2).

Furthermore, TFTs using oxide semiconductors have high field effectmobility. Therefore, driver circuits of display devices and the like canalso be formed using the TFTs.

REFERENCES Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

DISCLOSURE OF INVENTION

In the case where a plurality of circuits which are different from eachother is formed over an insulating surface, for example, when a pixelportion and a driver circuit are formed over one substrate, excellentswitching characteristics such as a high on-off ratio are needed for athin film transistor used for the pixel portion, and high operationspeed is needed for a thin film transistor used for the driver circuit.In particular, as the definition of a display device is higher, writingtime of a display image is reduced. Therefore, it is preferable that thethin film transistor used for the driver circuit operate at high speed.

An object of one embodiment of the present invention is to provide ahighly reliable thin film transistor with good electricalcharacteristics and a display device including the thin film transistoras a switching element.

One embodiment of the present invention is a semiconductor deviceincluding: a gate electrode layer over a substrate; a gate insulatinglayer over the gate electrode layer; an oxide semiconductor layer overthe gate insulating layer; an oxide insulating layer in contact withpart of the oxide semiconductor layer; a source electrode layer and adrain electrode layer each in contact with part of the oxidesemiconductor layer. In the oxide semiconductor layer, a region betweenthe source electrode layer and the oxide insulating layer and a regionbetween the drain electrode layer and the oxide insulating layer eachhave a thickness less than each of a region overlapping with the sourceelectrode layer, a region overlapping with the oxide insulating layer,and a region overlapping with the drain electrode layer.

Further, a superficial portion of the oxide semiconductor layer which isin contact with the oxide insulating layer has a crystal region.

In the above structure, the gate electrode layer, the source electrodelayer, and the drain electrode layer included in the semiconductordevice are formed using a film which contains a metal element selectedfrom aluminum, copper, molybdenum, titanium, chromium, tantalum,tungsten, neodymium, and scandium as its main component or a stackedfilm of alloy films containing any of the elements. Each of the sourceelectrode layer and the drain electrode layer is not limited to a singlelayer containing any of the above-described elements and may be a stackof two or more layers.

A light-transmitting oxide conductive layer of an indium oxide, an alloyof an indium oxide and a tin oxide, an alloy of an indium oxide and azinc oxide, a zinc oxide, a zinc aluminum oxide, a zinc aluminumoxynitride, a zinc gallium oxide, or the like can be used for the sourceelectrode layer, the drain electrode layer, and the gate electrodelayer, whereby a light-transmitting property of a pixel portion can beimproved and an aperture ratio can be increased.

The oxide conductive layer can be formed between the oxide semiconductorlayer and the film containing the metal element as its main component,which is for forming the source electrode layer and the drain electrodelayer, whereby a thin film transistor which has low contact resistanceand can operate at high speed can be formed.

In the above structure, the semiconductor device includes the oxidesemiconductor layer and an oxide insulating layer over the oxidesemiconductor layer. The oxide insulating layer in contact with thechannel formation region of the oxide semiconductor layer functions as achannel protective layer.

In the above structure, as the oxide insulating layer which functions asthe channel protective layer of the semiconductor device, an inorganicinsulating film formed by a sputtering method is used; typically, asilicon oxide film, a silicon nitride oxide film, an aluminum oxidefilm, an aluminum oxynitride film, or the like is used.

As the oxide semiconductor layer, a thin film of InMO₃(ZnO)_(m) (m>0 andm is not an integer) is formed. The thin film is used as an oxidesemiconductor layer to form a thin film transistor. Note that M denotesone metal element or a plurality of metal elements selected from Ga, Fe,Ni, Mn, and Co. As an example, M may be Ga or may contain the abovemetal element in addition to Ga, for example, M may be Ga and Ni or Gaand Fe. Moreover, in the oxide semiconductor, in some cases, atransition metal element such as Fe or Ni or an oxide of the transitionmetal is contained as an impurity element in addition to a metal elementcontained as M. In this specification, among the oxide semiconductorlayers whose composition formulas are represented by InMO₃(ZnO)_(m) (m>0and m is not an integer), an oxide semiconductor which contains Ga as Mis referred to as an In—Ga—Zn—O-based oxide semiconductor, and a thinfilm of the In—Ga—Zn—O-based oxide semiconductor is also referred to asan In—Ga—Zn—O-based film.

As a metal oxide used for the oxide semiconductor layer, any of thefollowing metal oxides can be used in addition to the above: anIn—Sn—O-based metal oxide; an In—Sn—Zn—O-based metal oxide; anIn—Al—Zn—O-based metal oxide; a Sn—Ga—Zn—O-based metal oxide; anAl—Ga—Zn—O-based metal oxide; a Sn—Al—Zn—O-based metal oxide; anIn—Zn—O-based metal oxide; a Sn—Zn—O-based metal oxide; an Al—Zn—O-basedmetal oxide; an In—O-based metal oxide; a Sn—O-based metal oxide; and aZn—O-based metal oxide. A silicon oxide may be contained in the oxidesemiconductor layer formed using the metal oxide.

For the oxide semiconductor layer, the one which is subjected todehydration or dehydrogenation at high temperature in a short time by anRTA method or the like is used. The heating process by an RTA method orthe like makes a superficial portion of the oxide semiconductor layerhave a crystal region including so-called nanocrystals with a grain sizeof greater than or equal to 1 nm and less than or equal to 20 nm and therest portion is amorphous or is formed of a mixture ofamorphousness/non-crystals and microcrystals, where an amorphous regionis dotted with microcrystals.

An oxide semiconductor layer having such a structure is used, wherebydeterioration of electrical characteristics due to a change to an n-typecaused by entry of moisture or elimination of oxygen to or from thesuperficial portion can be prevented. The superficial portion of theoxide semiconductor layer is on a back channel side and has a crystalregion including nanocrystals, so that generation of a parasitic channelcan be suppressed.

In the case where the oxide semiconductor layer is formed to have anisland shape after dehydration or dehydrogenation, no crystal region isformed in side surface portions. Although a crystal region is formedonly in a superficial portion, except for the side surface portions, anarea rate of the side surface portion is small and the above effect isnot prevented.

A display device can be formed using a driver circuit portion and apixel portion which are formed using thin film transistors each of whichis one embodiment of the present invention, over the same substrate, andan EL element, a liquid crystal element, an electrophoretic element, orthe like.

In the display device which is one embodiment of the present invention,a plurality of thin film transistors is provided in a pixel portion, andthe pixel portion has a region in which a gate electrode of one of thethin film transistors is connected to a source wiring or a drain wiringof another thin film transistor. In addition, in a driver circuit of thedisplay device which is one embodiment of the present invention, thereis a region where a gate electrode of a thin film transistor isconnected to a source wiring or a drain wiring of the thin filmtransistor.

Since a thin film transistor is easily broken due to static electricityor the like, a protective circuit for protecting the thin filmtransistor for the pixel portion is preferably provided over the samesubstrate for a gate line or a source line. The protective circuit ispreferably formed with a non-linear element including an oxidesemiconductor layer.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and an electronicappliance are all semiconductor devices.

In a thin film transistor including an oxide semiconductor layer, asuperficial portion of the oxide semiconductor layer includes a crystalregion in a channel formation region. Accordingly, a highly reliablethin film transistor and a highly reliable display device both with goodelectrical characteristics can be formed.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional view illustrating one embodiment of thepresent invention;

FIGS. 2A to 2E are cross-sectional process views illustrating oneembodiment of the present invention;

FIG. 3 is a top view illustrating one embodiment of the presentinvention;

FIGS. 4A1 and 4B1 are cross-sectional views and FIGS. 4A2 and 4B2 aretop views illustrating one embodiment of the present invention;

FIG. 5A is a cross sectional view and FIG. 5B is a top view illustratingone embodiment of the present invention;

FIGS. 6A to 6E are cross-sectional process views illustrating oneembodiment of the present invention;

FIGS. 7A and 7B are block diagrams of a semiconductor device;

FIGS. 8A and 8B are a circuit diagram and a timing chart of a signalline driver circuit, respectively;

FIGS. 9A to 9C are circuit diagrams illustrating a structure of a shiftregister;

FIGS. 10A and 10B are a circuit diagram and a timing chart illustratingoperation of a shift register;

FIGS. 11A1 and 11A2 are plan views and FIG. 11B is a cross-sectionalview illustrating one embodiment of the present invention;

FIG. 12 is a cross-sectional view illustrating one embodiment of thepresent invention;

FIG. 13 is a cross-sectional view illustrating one embodiment of thepresent invention;

FIG. 14 is a view illustrating an equivalent circuit of a pixel of asemiconductor device;

FIGS. 15A to 15C are cross-sectional views each illustrating oneembodiment of the present invention;

FIG. 16A is a plan view and FIG. 16B is a cross-sectional viewillustrating one embodiment of the present invention;

FIGS. 17A and 17B are views illustrating examples of usage patterns ofelectronic paper;

FIG. 18 is an external view of one example of an electronic book reader;

FIGS. 19A and 19B are external views illustrating examples of atelevision device and a digital photo frame, respectively;

FIGS. 20A and 20B are external views illustrating examples of gamemachines;

FIGS. 21A and 21B are external views illustrating examples of mobilephones;

FIGS. 22A to 22D are cross-sectional views each illustrating oneembodiment of the present invention;

FIG. 23 is a view illustrating an example of a crystal structure of anoxide semiconductor;

FIG. 24 is a diagram briefly illustrating scientific computing;

FIGS. 25A and 25B are diagrams briefly illustrating scientificcomputing; and

FIGS. 26A and 26B are graphs showing results of scientific computing.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described with reference to drawings. Note that thepresent invention is not limited to the following description, and itwill be easily understood by those skilled in the art that the modes anddetails of the present invention can be modified in various ways withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention should not be construed as being limited to thefollowing description of the embodiments. Note that in structures of thepresent invention described below, the same portions or portions havingsimilar functions are denoted by the same reference numerals indifferent drawings, and description thereof is omitted.

Embodiment 1

In this embodiment, a structure of a thin film transistor will bedescribed with reference to FIG. 1.

A channel protected thin film transistor of this embodiment isillustrated in FIG. 1.

In a thin film transistor 470 illustrated in FIG. 1, over a substrate400 having an insulating surface, a gate electrode layer 421 a, a gateinsulating layer 402, an oxide semiconductor layer 423 including achannel formation region, a source electrode layer 425 a, a drainelectrode layer 425 b, and an oxide insulating layer 426 a functioningas a channel protective layer are provided.

The gate electrode layer 421 a can be formed with a single-layerstructure or a stacked-layer structure using any of metal materials suchas aluminum, copper, molybdenum, titanium, chromium, tantalum, tungsten,neodymium, and scandium; an alloy material containing any of these metalmaterials as its main component; or a nitride containing any of thesemetal materials. It is preferable that the gate electrode layer beformed with the use of a low-resistance metal material such as aluminumor copper, which is effective, though it is to be noted that thelow-resistance metal material is preferably used in combination with arefractory metal material because it has disadvantages such as low heatresistance and a tendency to be corroded. As the refractory metalmaterial, molybdenum, titanium, chromium, tantalum, tungsten, neodymium,scandium, or the like can be used.

Further, in order to increase the aperture ratio of a pixel portion, alight-transmitting oxide conductive layer of an indium oxide, an alloyof an indium oxide and a tin oxide, an alloy of an indium oxide and azinc oxide, a zinc oxide, a zinc aluminum oxide, a zinc aluminumoxynitride, a zinc gallium oxide, or the like may be used as the gateelectrode layer 421 a.

As the gate insulating layer 402, a single-layer film or a laminate filmof any of a silicon oxide, a silicon oxynitride, a silicon nitrideoxide, a silicon nitride, an aluminum oxide, a tantalum oxide, and thelike formed by a CVD method, a sputtering method, or the like can beused.

The oxide semiconductor layer 423 is formed using an In—Ga—Zn—O-basedfilm which contains In, Ga, and Zn and has a structure represented asInMO₃(ZnO)_(m) (m>0). Note that M denotes one or more of metal elementsselected from gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), andcobalt (Co). As an example, M may be Ga or may contain the above metalelement in addition to Ga, for example, M may be Ga and Ni or Ga and Fe.Moreover, in the oxide semiconductor, in some cases, a transition metalelement such as Fe or Ni or an oxide of the transition metal iscontained as an impurity element in addition to a metal elementcontained as M.

The oxide semiconductor layer 423 is formed by a sputtering method to athickness greater than or equal to 10 nm and less than or equal to 300nm, preferably greater than or equal to 20 nm and less than or equal to100 nm. Note that as illustrated in FIG. 1, in the oxide semiconductorlayer 423, a third region 424 c between the source electrode layer 425 aand the oxide insulating layer 426 a, and a fourth region 424 d betweenthe drain electrode layer 425 b and the oxide insulating layer 426 aeach have a thickness less than each of a first region 424 a overlappingwith the source electrode layer 425 a, a fifth region 424 e overlappingwith the oxide insulating layer 426 a, and a second region 424 boverlapping with the drain electrode layer 425 b.

As the oxide semiconductor layer 423, the one subjected to dehydrationor dehydrogenation at high temperature for a short time by a rapidthermal annealing (RTA) method or the like is used. Dehydration ordehydrogenation can be performed by an RTA treatment with the use of ahigh-temperature gas (an inert gas such as nitrogen or a rare gas) orlight at a temperature higher than or equal to 500° C. and lower than orequal to 750° C. (or a temperature lower than or equal to the strainpoint of a glass substrate) for approximately greater than or equal toone minute and less than or equal to ten minutes, preferably at 650° C.for approximately greater than or equal to three minutes and less thanor equal to six minutes. By an RTA method, dehydration ordehydrogenation can be performed in a short time; therefore, a treatmentcan be performed even at a temperature higher than the strain point of aglass substrate.

The oxide semiconductor layer 423 is an amorphous layer having manydangling bonds at the stage where the oxide semiconductor layer 423 isformed. Through a heating step for the dehydration or dehydrogenation,dangling bonds within a short distance are bonded to each other, so thatthe oxide semiconductor layer 423 can have an ordered amorphousstructure. As ordering proceeds, the oxide semiconductor layer 423 comesto be formed of a mixture of amorphousness/non-crystals andmicrocrystals, where an amorphous region is dotted with microcrystals,or be formed of amorphousness/non-crystals. Here, a microcrystal is aso-called nanocrystal with a particle size of greater than or equal to 1nm and less than or equal to 20 nm, which is smaller than that of amicrocrystalline particle generally called a microcrystal.

It is preferable that the superficial portion of the oxide semiconductorlayer 423 in the fifth region 424 e overlapping with the oxideinsulating layer 426 a include a crystal region and nanocrystals c-axisoriented in a direction perpendicular to a surface of the oxidesemiconductor layer be formed in the crystal region. In that case, thelong axis is in the c-axis direction and the length in the short-axisdirection is greater than or equal to 1 nm and less than or equal to 20nm.

By using an oxide semiconductor layer which has such a structure,deterioration of electrical characteristics due to a change to an n-typecaused by entry of moisture or elimination of oxygen to or from thesuperficial portion can be prevented because a dense crystal regionincluding nanocrystals exists in the superficial portion of the channelformation region. Further, since the superficial portion of the oxidesemiconductor layer in the channel formation region is on the backchannel side, preventing the oxide semiconductor layer from beingchanged to an n-type is also effective for suppression of generation ofa parasitic channel.

Here, a crystal structure of an In—Ga—Zn—O-based film, which is likelyto grow, depends on a used metal oxide target. For example, in the casewhere an In—Ga—Zn—O-based film is formed using a metal oxide target,which contains In, Ga, and Zn so that the molar ratio of In₂O₃ to Ga₂O₃to ZnO is 1:1:0.5, and crystallization is performed through a heatingstep, a hexagonal system layered compound crystal structure in which oneoxide layer or two oxide layers containing Ga and Zn are mixed betweenIn oxide layers is likely to be formed. At this time, a crystal regionis likely to have a crystal structure represented by In₂Ga₂ZnO₇ (seeFIG. 23). The molar ratio of In to Ga to Zn in the structure of theamorphous region or the region in which amorphousness/non-crystals andmicrocrystals are mixed in the oxide semiconductor layer is likely to be1:1:0.5. Alternatively, in the case where deposition is performed usinga metal oxide semiconductor target, whose molar ratio of In₂O₃ to Ga₂O₃to ZnO is 1:1:1, and crystallization is performed through a heatingstep, an oxide layer containing Ga and Zn interposed between In oxidelayers is likely to have a two-layer structure. Since the crystalstructure of the oxide layer containing Ga and Zn of the latter having atwo-layer structure is stable and thus crystal growth is likely tooccur, in the case where a target whose molar ratio of In₂O₃ to Ga₂O₃ toZnO is 1:1:1 is used, and crystallization is performed through a heatingstep, a crystal continuous from an outer layer to an interface between agate insulating layer and the oxide layer containing Ga and Zn is formedin some cases. Note that the molar ratio may be referred to as the ratioof atoms.

In this embodiment, the source electrode layer 425 a and the drainelectrode layer 425 b each have a three-layer structure of a firstconductive layer, a second conductive layer, and a third conductivelayer. As materials of these layers, materials each similar to that ofthe gate electrode layer 421 a can be appropriately used.

Further, the light-transmitting oxide conductive layer is used for thesource and drain electrode layers 425 a and 425 b in a manner similar tothat of the gate electrode layer 421 a, whereby a light-transmittingproperty of the pixel portion can be improved and the aperture ratio canalso be increased.

Further, the oxide conductive layer may be formed between the oxidesemiconductor layer 423 and the film containing any of the above metalmaterials as its main component, which is to be the source and drainelectrode layers 425 a and 425 b, so that contact resistance can bereduced.

Over the oxide semiconductor layer 423, the oxide insulating layer 426 afunctioning as a channel protective layer is provided in contact withthe oxide semiconductor layer 423. The oxide insulating layer 426 a isformed by a sputtering method using an inorganic insulating film,typically a silicon oxide film, a silicon nitride oxide film, analuminum oxide film, an aluminum oxynitride film, or the like.

In FIG. 1, the channel formation region refers to the fifth region 424 ewhere the oxide insulating layer 426 a functioning as a channelprotective layer overlaps with the gate electrode layer with the gateinsulating layer 402 interposed therebetween. Note that channel length Lof a thin film transistor is defined as a distance between a sourceelectrode layer and a drain electrode layer; however, in the case of thechannel protected thin film transistor 470, channel length L is equal tothe width of the oxide insulating layer 426 a which is in a directionparallel to a direction in which carriers flow. Note also that thechannel length L of the thin film transistor 470 means the length of theoxide semiconductor layer 423 at the interface with the oxide insulatinglayer 426 a, i.e., the base of a trapezoid which represents the oxideinsulating layer 426 a in the cross-sectional view of FIG. 1.

In a channel protected thin film transistor, when a source electrodelayer and a drain electrode layer are provided over an oxide insulatinglayer having a small width which is reduced so as to shorten the channellength L of a channel formation region, a short circuit between thesource electrode layer and the drain electrode layer could be formedover the oxide insulating layer. In order to solve this problem, thesource electrode layer 425 a and the drain electrode layer 425 b areprovided so that their end portions are apart from the oxide insulatinglayer 426 a with a reduced width in the thin film transistor of FIG. 1.As the channel protected thin film transistor 470, the width of theoxide insulating layer can be reduced such that the channel length L ofthe channel formation region becomes as short as a length greater thanor equal to 0.1 μm and less than or equal to 2 μm, whereby a thin filmtransistor having high operation speed is achieved.

Hereinafter, an example of a manufacturing process of a display devicewhich includes the channel protected thin film transistor illustrated inFIG. 1 is described with reference to FIGS. 2A to 2E and FIG. 3. Notethat FIG. 3 is a plan view of the display device and each of FIGS. 2A to2E is a cross-sectional view taken along line A1-A2 and line B1-B2 ofFIG. 3.

First, the substrate 400 is prepared. As the substrate 400, any of thefollowing substrates can be used: non-alkaline glass substrates made ofbarium borosilicate glass, aluminoborosilicate glass, aluminosilicateglass, and the like by a fusion method or a float method; ceramicsubstrates; plastic substrates having heat resistance enough towithstand a process temperature of this manufacturing process; and thelike. Alternatively, a metal substrate such as a stainless steel alloysubstrate having a surface provided with an insulating film may be used.

Note that instead of the glass substrate described above, a substrateformed using an insulator, such as a ceramic substrate, a quartzsubstrate, or a sapphire substrate, may be used as the substrate 400.Alternatively, a crystallized glass substrate or the like can be used.

Further, as a base film, an insulating film may be formed over thesubstrate 400. As the base film, any of a silicon oxide film, a siliconnitride film, a silicon oxynitride film, and a silicon nitride oxidefilm may be formed to have a single-layer structure or a stacked-layerstructure by a CVD method, a sputtering method, or the like. In the casewhere a substrate containing mobile ions, such as a glass substrate, isused as the substrate 400, a film containing nitrogen, such as a siliconnitride film or a silicon nitride oxide film, is used as the base film,whereby the mobile ions can be prevented from entering the oxidesemiconductor layer or the semiconductor layer.

Next, a conductive film for forming a gate wiring including the gateelectrode layer 421 a, a capacitor wiring 421 b, and a first terminal421 c is formed over an entire surface of the substrate 400 by asputtering method or a vacuum evaporation method. Next, after theformation of the conductive film over the entire surface of thesubstrate 400, in a first photolithography step, a resist mask is formedand an unnecessary portion is removed by etching to form wirings and anelectrode (the gate wiring including the gate electrode layer 421 a, thecapacitor wiring 421 b, and the first terminal 421 c). At this time,etching is preferably performed so that at least an end portion of thegate electrode layer 421 a is tapered, in order to preventdisconnection.

The gate wiring including the gate electrode layer 421 a, the capacitorwiring 421 b, and the first terminal 421 c in a terminal portion can beformed with a single-layer structure or a stacked-layer structure usingany of metal materials such as aluminum, copper, molybdenum, titanium,chromium, tantalum, tungsten, neodymium, and scandium; an alloy materialcontaining any of these metal materials as its main component; or anitride containing any of these metal materials. It is preferable thatthe gate electrode layer be formed with the use of a low-resistancemetal material such as aluminum or copper, which is effective, though itis to be noted that the low-resistance metal material is preferably usedin combination with a refractory metal material because it hasdisadvantages such as low heat resistance and a tendency to be corroded.As the refractory metal material, molybdenum, titanium, chromium,tantalum, tungsten, neodymium, scandium, or the like can be used.

For example, as a stacked-layer structure of the gate electrode layer421 a, the following structures are preferable: a two-layer structure inwhich a molybdenum layer is stacked over an aluminum layer; a two-layerstructure in which a molybdenum layer is stacked over a copper layer; atwo-layer structure in which a titanium nitride layer or a tantalumnitride layer is stacked over a copper layer; and a two-layer structureof a titanium nitride layer and a molybdenum layer. As a three-layerstructure, the following structure is preferable: a stacked-layerstructure containing aluminum, an alloy of aluminum and silicon, analloy of aluminum and titanium, or an alloy of aluminum and neodymium ina middle layer and any of tungsten, tungsten nitride, titanium nitride,and titanium in a top layer and a bottom layer.

At that time, a light-transmitting oxide conductive layer may be usedfor one or more of the electrode layers and the wiring layers toincrease the aperture ratio. For example, the oxide conductive layer canbe formed using an indium oxide, an alloy of an indium oxide and a tinoxide, an alloy of an indium oxide and a zinc oxide, a zinc oxide, azinc aluminum oxide, a zinc aluminum oxynitride, a zinc gallium oxide,or the like.

Next, the gate insulating layer 402 is formed so as to cover the gateelectrode layer 421 a (see FIG. 2A). The gate insulating layer 402 isformed to a thickness greater than or equal to 10 nm and less than orequal to 400 nm by a CVD method, a sputtering method, or the like.

For example, as the gate insulating layer 402, a silicon oxide film witha thickness of 100 nm is formed by a CVD method, a sputtering method, orthe like. Needless to say, the gate insulating layer 402 is not limitedto such a silicon oxide film and may be formed to have a single-layerstructure or a stacked-layer structure using any other insulating filmssuch as a silicon oxynitride film, a silicon nitride oxide film, asilicon nitride film, an aluminum oxide film, and a tantalum oxide film.

The gate insulating layer 402 is formed using a high-density plasmaapparatus. Here, a high-density plasma apparatus refers to an apparatuswhich can realize a plasma density greater than or equal to 1×10¹¹/cm³.For example, plasma is generated by applying a microwave power of 3 kWto 6 kW so that the insulating film is formed.

A monosilane gas (SiH₄), nitrous oxide (N₂O), and a rare gas areintroduced into a chamber as a source gas to generate high-densityplasma at a pressure greater than or equal to 10 Pa and less than orequal to 30 Pa so that an insulating film is formed over a substratehaving an insulating surface, such as a glass substrate. After that, thesupply of a monosilane gas is stopped, and nitrous oxide (N₂O) and arare gas are introduced without exposure to the air, so that a surfaceof the insulating film is subjected to a plasma treatment. The plasmatreatment performed on the surface of the insulating film by introducingnitrous oxide (N₂O) and a rare gas is performed at least after theinsulating film is formed. The insulating film formed through the aboveprocess procedure has a small thickness and corresponds to an insulatingfilm whose reliability can be ensured even when it has a thickness lessthan 100 nm, for example.

In forming the gate insulating layer 402, the flow ratio of a monosilanegas (SiH₄) to nitrous oxide (N₂O) which are introduced into the chamberis in the range of 1:10 to 1:200. In addition, as a rare gas which isintroduced into the chamber, helium, argon, krypton, xenon, or the likecan be used. In particular, argon, which is inexpensive, is preferablyused.

In addition, since the insulating film formed by using the high-densityplasma apparatus can have a certain thickness, the insulating film hasexcellent step coverage. Further, as for the insulating film formedusing the high-density plasma apparatus, the thickness of a thin filmcan be controlled precisely.

Unlike an insulating film formed using a conventional parallel plateplasma enhanced CVD apparatus in many points, the insulating film formedthrough the above process procedure has an etching rate which is lowerthan that of the insulating film formed using the conventional parallelplate plasma enhanced CVD apparatus by greater than or equal to 10% orgreater than or equal to 20% in the case where the etching rates withthe same etchant are compared with each other. Thus, it can be said thatthe insulating film obtained using a high-density plasma apparatus is adense film.

Alternatively, the gate insulating layer 402 can be formed using asilicon oxide layer by a CVD method in which an organosilane gas isused. As an organosilane gas, a silicon-containing compound such astetraethoxysilane (TEOS) (chemical formula: Si(OC₂H₅)₄),tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄),tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (chemical formula:SiH(OC₂H₅)₃), or trisdimethylaminosilane (chemical formula:SiH(N(CH₃)₂)₃) can be used.

Alternatively, the gate insulating layer 402 may be formed using onekind of an oxide, a nitride, an oxynitride, and a nitride oxide ofaluminum, yttrium, or hafnium; or a compound containing at least two ormore kinds of the above.

Note that in this specification, the term “oxynitride” refers to asubstance that contains oxygen atoms and nitrogen atoms so that thenumber of the oxygen atoms is larger than that of the nitrogen atoms andthe term “nitride oxide” refers to a substance that contains nitrogenatoms and oxygen atoms so that the number of the nitrogen atoms islarger than that of the oxygen atoms. For example, a “silicon oxynitridefilm” means a film that contains oxygen atoms and nitrogen atoms so thatthe number of the oxygen atoms is larger than that of the nitrogen atomsand, in the case where measurements are performed using Rutherfordbackscattering spectrometry (RBS) and hydrogen forward scattering (HFS),contains oxygen, nitrogen, silicon, and hydrogen at concentrationsranging from 50 atomic % to 70 atomic %, 0.5 atomic % to 15 atomic %, 25atomic % to 35 atomic %, and 0.1 atomic % to 10 atomic %, respectively.Further, a “silicon nitride oxide film” means a film that containsnitrogen atoms and oxygen atoms so that the number of the nitrogen atomsis larger than that of the oxygen atoms and, in the case wheremeasurements are performed using RBS and HFS, contains oxygen, nitrogen,silicon, and hydrogen at concentrations ranging from 5 atomic % to 30atomic %, 20 atomic % to 55 atomic %, 25 atomic % to 35 atomic %, and 10atomic % to 30 atomic %, respectively. Note that percentages ofnitrogen, oxygen, silicon, and hydrogen fall within the ranges givenabove, where the total number of atoms contained in the siliconoxynitride film or the silicon nitride oxide film is defined as 100 at.%.

Note that before an oxide semiconductor film for forming the oxidesemiconductor layer 423 is formed, dust on a surface of the gateinsulating layer is preferably removed by performing reverse sputteringin which an argon gas is introduced and plasma is generated. The reversesputtering refers to a method in which, without application of voltageto a target side, an RF power source is used for application of voltageto a substrate side in an argon atmosphere so that plasma is generatedaround the substrate to modify a surface. Note that instead of an argonatmosphere, a nitrogen atmosphere, a helium atmosphere, or the like maybe used. Alternatively, an argon atmosphere to which oxygen, N₂O, or thelike is added may be used. Still alternatively, an argon atmosphere towhich Cl₂, CF₄, or the like is added may be used. After the reversesputtering, the oxide semiconductor film is formed without exposure tothe air, whereby adhesion of particles (dust) and moisture to aninterface between the gate insulating layer 402 and the oxidesemiconductor layer 423 can be prevented.

Next, an oxide semiconductor film is formed over the gate insulatinglayer 402 to a thickness greater than or equal to 5 nm and less than orequal to 200 nm, preferably greater than or equal to 10 nm and less thanor equal to 40 nm.

As the oxide semiconductor film, any of the following oxidesemiconductor films can be applied: an In—Ga—Zn—O-based oxidesemiconductor film; an In—Sn—Zn—O-based oxide semiconductor film; anIn—Al—Zn—O-based oxide semiconductor film; a Sn—Ga—Zn—O-based oxidesemiconductor film; an Al—Ga—Zn—O-based oxide semiconductor film; aSn—Al—Zn—O-based oxide semiconductor film; an In—Zn—O-based oxidesemiconductor film; a Sn—Zn—O-based oxide semiconductor film; anAl—Zn—O-based oxide semiconductor film; an In—O-based oxidesemiconductor film; a Sn—O-based oxide semiconductor film; and aZn—O-based oxide semiconductor film. Alternatively, the oxidesemiconductor film can be formed by a sputtering method in a rare gas(typically argon) atmosphere, an oxygen atmosphere, or an atmosphere ofa rare gas (typically argon) and oxygen. In the case of using asputtering method, film deposition may be performed using a targetcontaining SiO₂ at greater than or equal to 2 percent by weight and lessthan or equal to 10 percent by weight and SiOx (x>0) which inhibitscrystallization may be contained in the oxide semiconductor film.

Here, the oxide semiconductor film is formed using a metal oxide target,which contains In, Ga, and Zn (the molar ratio of In₂O₃ to Ga₂O₃ to ZnOis 1:1:0.5, or the molar ratio of In to Ga to ZnO is 1:1:1 or 1:1:2),under conditions where the distance between the substrate and the targetis 100 mm, the pressure is 0.6 Pa, and the direct current (DC) power is0.5 kW, and the atmosphere is an oxygen atmosphere (the proportion ofthe oxygen flow is 100%). It is preferable that a pulse direct current(DC) power source is used because powder substances (also referred to asparticles or dust) generated in film deposition can be reduced and thefilm thickness can be uniform. In this embodiment, as the oxidesemiconductor film, a 30-nm-thick In—Ga—Zn—O-based film is formed by asputtering method with the use of an In—Ga—Zn—O-based metal oxidetarget.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used as a sputtering power source, a DCsputtering method in which DC power source is used, and a pulsed DCsputtering method in which a bias is applied in a pulsed manner. An RFsputtering method is mainly used in the case where an insulating film isformed, and a DC sputtering method is mainly used in the case where ametal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering, and asputtering apparatus used for an ECR sputtering in which plasmagenerated with the use of microwaves is used without using glowdischarge.

Furthermore, as a deposition method using a sputtering method, there arealso a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringdeposition to form a thin compound film thereof, and a bias sputteringmethod in which voltage is also applied to a substrate duringdeposition.

Next, in a second photolithography step, a resist mask is formed and theIn—Ga—Zn—O-based film is etched. In the etching, organic acid such ascitric acid or oxalic acid can be used for an etchant. Here, theIn—Ga—Zn—O-based film is etched by wet etching with the use of ITO-07N(manufactured by Kanto Chemical Co., Inc.) to remove an unnecessaryportion. Thus, the In—Ga—Zn—O-based film is processed to have an islandshape, whereby the oxide semiconductor layer 423 is formed. The endportions of the oxide semiconductor layer 423 are etched to have taperedshapes, whereby breakage of a wiring due to a step shape can beprevented. Note that etching here is not limited to wet etching and dryetching may be performed.

Then, the oxide semiconductor layer is subjected to dehydration ordehydrogenation. A first heat treatment for the dehydration ordehydrogenation can be performed by a rapid thermal annealing (RTA)treatment with the use of a high-temperature gas (an inert gas such asnitrogen or a rare gas) or light at a temperature higher than or equalto 500° C. and lower than or equal to 750° C. (or a temperature lowerthan or equal to the strain point of a glass substrate) forapproximately greater than or equal to one minute and less than or equalto ten minutes, preferably at 650° C. for approximately greater than orequal to three minutes and less than or equal to six minutes. By an RTAmethod, dehydration or dehydrogenation can be performed in a short time;therefore, a treatment can be performed even at a temperature higherthan the strain point of a glass substrate. Note that the timing of theheat treatment is not limited to this timing and may be performed pluraltimes, for example, before and after a photolithography step or adeposition step.

Here, the superficial portion of the oxide semiconductor layer 423 iscrystallized by the first heat treatment and thus comes to have thecrystal region 106 including nanocrystals. The rest portion of the oxidesemiconductor layer 423 comes to be amorphous or be formed of a mixtureof amorphousness/non-crystals and microcrystals, where an amorphousregion is dotted with microcrystals. Note that the crystal region 106 ispart of the oxide semiconductor layer 423 and hereinafter, the “oxidesemiconductor layer 423” includes the crystal region 106.

Note that in this specification, a heat treatment in the atmosphere ofan inert gas such as nitrogen or a rare gas is referred to as a heattreatment for dehydration or dehydrogenation. In this specification,“dehydration” or “dehydrogenation” does not indicate elimination of onlyH₂ or H₂O by a heat treatment. For convenience, elimination of H, OH,and the like is referred to as “dehydration or dehydrogenation”.

In addition, when the temperature is lowered from a heating temperatureT at which the oxide semiconductor layer is subjected to the dehydrationor dehydrogenation, it is important to prevent entry of water orhydrogen by using the same furnace that has been used for thedehydration or dehydrogenation, in such a manner that the oxidesemiconductor layer is not exposed to the air. When a thin filmtransistor is formed using an oxide semiconductor layer obtained bychanging an oxide semiconductor layer into a low-resistance oxidesemiconductor layer, i.e., an n-type (e.g., n⁻-type or n⁺-type) oxidesemiconductor layer by performing dehydration or dehydrogenation and bychanging the low-resistance oxide semiconductor layer into ahigh-resistance oxide semiconductor layer so that the oxidesemiconductor layer becomes an i-type oxide semiconductor layer, thethreshold voltage of the thin film transistor is positive, so that aswitching element having a so-called normally-off property can berealized. It is preferable for a display device that a channel be formedwith positive threshold voltage that is as close to 0 V as possible in athin film transistor. If the threshold voltage of the thin filmtransistor is negative, it tends to have a so-called normally-onproperty; in other words, current flows between the source electrode andthe drain electrode even when the gate voltage is 0 V. In an activematrix display device, electrical characteristics of a thin filmtransistor included in a circuit are important and the performance ofthe display device depends on the electrical characteristics. Inparticular, of the electrical characteristics of the thin filmtransistor, the threshold voltage (V_(th)) is important. When thethreshold voltage value is high or is on the minus side even when thefield effect mobility is high, it is difficult to control the circuit.In the case where a thin film transistor has a high threshold voltageand a large absolute value of its threshold voltage, the thin filmtransistor cannot perform a switching function as the TFT and might be aload when the thin film transistor is driven at low voltage. In the caseof an n-channel thin film transistor, it is preferable that a channel beformed and drain current flows after positive voltage is applied as gatevoltage. A transistor in which a channel is not formed unless drivingvoltage is raised and a transistor in which a channel is formed anddrain current flows even when negative voltage is applied are unsuitablefor a thin film transistor used in a circuit.

In addition, the gas atmosphere in which the temperature is lowered fromthe heating temperature T may be switched to a gas atmosphere which isdifferent from the gas atmosphere in which the temperature is raised tothe heating temperature T. For example, cooling is performed in thefurnace where the heat treatment for dehydration or dehydrogenation isperformed while the furnace is filled with a high-purity oxygen gas, ahigh-purity N₂O gas, or ultra-dry air (having a dew point lower than orequal to −40° C., preferably lower than or equal to −60° C.) withoutexposure to the air.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the atmosphere.Alternatively, the purity of an inert gas which is introduced into aheat treatment apparatus is preferably greater than or equal to 6N(99.9999%), more preferably greater than or equal to 7N (99.99999%)(that is, the impurity concentration is less than or equal to 1 ppm,preferably less than or equal to 0.1 ppm).

In the case where the heat treatment is performed in an inert gasatmosphere, the oxide semiconductor layer is changed into anoxygen-deficient oxide semiconductor layer so that the oxidesemiconductor layer becomes a low-resistance oxide semiconductor layer(i.e., an n-type (e.g., n⁻-type) oxide semiconductor layer) through theheat treatment. After that, the oxide semiconductor layer is made to bein an oxygen excess state by the formation of an oxide insulating layerwhich is in contact with the oxide semiconductor layer. Thus, the oxidesemiconductor layer is made to be i-type; that is, the oxidesemiconductor layer is changed into a high-resistance oxidesemiconductor layer. Accordingly, it is possible to form a highlyreliable thin film transistor having favorable electricalcharacteristics.

Depending on a condition of the first heat treatment or a material ofthe oxide semiconductor layer, the oxide semiconductor layer may bepartly crystallized. By the first heat treatment, the oxidesemiconductor layer 423 is changed to an oxygen-deficient type and theresistance thereof is reduced. After the first heat treatment, thecarrier concentration is higher than that of the oxide semiconductorfilm just after the film deposition, so that the oxide semiconductorlayer has a carrier concentration of, preferably, greater than or equalto 1×10¹⁸/cm³.

The first heat treatment for the oxide semiconductor layer may beperformed before the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layer. In that case, the substrate istaken out of the heat treatment apparatus and a second photolithographystep is performed after the first heat treatment. A crystal region isnot formed in a side surface portion of the oxide semiconductor layer423 and the crystal region 106 is formed only in an upper layer portionof the oxide semiconductor layer 423.

Next, in a third photolithography step, a resist mask is formed and anunnecessary portion is removed by etching to form a contact holereaching the wiring or the electrode layer which is formed using thesame material as the gate electrode layer 421 a (see FIG. 2B). Thiscontact hole is provided for direct connection with a conductive film tobe formed later. For example, a contact hole is formed when a thin filmtransistor whose gate electrode layer is in direct contact with thesource or drain electrode layer in the driver circuit portion is formed,or when a terminal that is electrically connected to a gate wiring of aterminal portion is formed.

Next, an oxide insulating film is formed over the oxide semiconductorlayer 423 and the gate insulating layer 402 by a sputtering method;then, in a fourth photolithography step, a resist mask is formed andselective etching is performed thereon so as to form the oxideinsulating layers 426 a, 426 b, 426 c, and 426 d. After that, the resistmask is removed (see FIG. 2C). At this stage, a region which is incontact with the oxide insulating layer 426 a is formed in the oxidesemiconductor layer. Within this region, a region that overlaps with thegate electrode layer with the gate insulating layer interposedtherebetween and also overlaps with the oxide insulating layer 426 a isthe channel formation region. Further, in the fourth photolithographystep, a contact hole reaching the first terminal 421 c is also formed.

The oxide insulating film can be formed to a thickness at least greaterthan or equal to 1 nm by a method by which impurities such as water andhydrogen are not mixed into the oxide insulating film, such as asputtering method, as appropriate. In this embodiment, a silicon oxidefilm is formed by a sputtering method, as the oxide insulating film. Thesubstrate temperature in the film deposition may be higher than or equalto room temperature and lower than or equal to 300° C.; in thisembodiment, the substrate temperature is 100° C. The deposition of thesilicon oxide film by a sputtering method can be performed in a rare gas(typically, argon) atmosphere, an oxygen atmosphere, or an atmosphere ofa rare gas (typically, argon) and oxygen. As a target, a silicon oxidetarget or a silicon target can be used. For example, with the use of asilicon target, a silicon oxide film can be formed by a sputteringmethod in an atmosphere of oxygen and a rare gas. As the oxideinsulating film formed in contact with the oxide semiconductor layerwhose resistance is reduced, an inorganic insulating film that does notinclude impurities such as water, a hydrogen ion, and OH⁻ and blocksentry of these from the outside is used. Typically, a silicon oxidefilm, a silicon nitride oxide film, an aluminum oxide film, an aluminumoxynitride film, or the like is used.

In this embodiment, the film deposition is performed by a pulsed DCsputtering method using a columnar polycrystalline, boron-doped silicontarget which has a purity of 6N (with a resistivity of 0.01 Ω·cm), inwhich the distance between the substrate and the target (T-S distance)is 89 mm, the pressure is 0.4 Pa, the direct-current (DC) power sourceis 6 kW, and the atmosphere is oxygen (the oxygen flow rate is 100%).The film thickness thereof is 300 nm.

Next, a conductive film is formed using a metal material over the oxidesemiconductor layer 423 by a sputtering method, a vacuum evaporationmethod, or the like. As a material of the conductive film, a materialsimilar to that of the gate electrode layer 421 a can be used.

In this embodiment, a conductive film in which first to third conductivefilms are stacked is formed. For example, the first conductive film andthe third conductive film are formed using titanium that is aheat-resistant conductive material, and the second conductive film isformed using an aluminum alloy containing neodymium. Such a structurecan reduce generation of hillock and utilize a low resistance propertyof aluminum. Although a three-layer structure of the first to thirdconductive films is used in this embodiment, one embodiment of thepresent invention is not limited to this. A single-layer structure, atwo-layer structure, or a stacked-layer structure of four or more layersmay be employed. A structure including two layers or four or more layersmay be employed. For example, a single-layer structure of a titaniumfilm or a single-layer structure of an aluminum film containing siliconmay be employed.

Note that at the time of deposition of the conductive film on and incontact with the oxide semiconductor layer, whose superficial portionhas the dense crystal region 106 including nanocrystals, damage on thecrystal region due to the deposition or heat by the deposition makes thecrystal region 106 of the semiconductor layer amorphous in some cases.However, in the manufacturing method of a thin film transistor which isdescribed in this embodiment, the oxide insulating layer 426 afunctioning as a channel protective layer is provided in contact withthe region to be the channel formation region of the oxide semiconductorlayer, whereby the superficial portion of the oxide semiconductor layer423 at least in the channel formation region (the fifth region) can havethe crystal region 106.

Next, in a fifth photolithography step, a resist mask is formed andunnecessary portions are removed by etching so as to form the source anddrain electrode layers 425 a and 425 b and a connection electrode 429.Wet etching or dry etching is employed as an etching method at thistime. For example, when the first conductive film and the thirdconductive film are formed using titanium and the second conductive filmis formed using an aluminum alloy containing neodymium, wet etching canbe performed using a hydrogen peroxide solution or heated hydrochloricacid as an etchant.

By this etching step, the oxide semiconductor layer 423 is partly etchedso that the third region 424 c between the source electrode layer 425 aand the oxide insulating layer 426 a, and the fourth region 424 dbetween the drain electrode layer 425 b and the oxide insulating layer426 a each have a thickness less than each of the first region 424 aoverlapping with the source electrode layer 425 a, the fifth region 424e overlapping with the oxide insulating layer 426 a, and the secondregion 424 b overlapping with the drain electrode layer 425 b (see FIG.2D). Note that the fifth region 424 e of the oxide semiconductor layer423 is protected by the oxide insulating layer 426 a so as not to beetched and thus the superficial portion has a dense crystal regionincluding nanocrystals at least in the channel formation region. In thechannel formation region, the superficial portion of the oxidesemiconductor layer is on the back channel side and the crystal regioncan suppress generation of a parasitic channel.

In addition, by this fifth photolithography step, the connectionelectrode 429 is directly connected to the first terminal 421 c of theterminal portion through a contact hole formed in the gate insulatinglayer. Although not illustrated, a source or drain wiring and a gateelectrode of a thin film transistor of a driver circuit are directlyconnected to each other through the same steps as the above steps.

Next, an oxide insulating layer 428 which covers the thin filmtransistor 470 is formed (see FIG. 2E). As the oxide insulating layer428, an oxide insulating layer formed using a silicon oxide film, asilicon oxynitride film, an aluminum oxide film, an aluminum oxynitridefilm, or a tantalum oxide film which is formed by a sputtering method orthe like.

The oxide insulating layer can be formed by a method by which impuritiessuch as water and hydrogen are not mixed into the oxide insulatinglayer, such as a sputtering method, as appropriate. In this embodiment,a silicon oxide film is formed by a sputtering method, for the oxideinsulating layer. The substrate temperature in the film deposition maybe higher than or equal to room temperature and lower than or equal to300° C.; in this embodiment, the substrate temperature is 100° C. Inorder to prevent entry of impurities such as water and hydrogen in thefilm deposition, pre-baking is preferably performed under reducedpressure at a temperature higher than or equal to 150° C. and lower thanor equal to 350° C. for greater than or equal to two minutes and lessthan or equal to ten minutes before the film deposition, to form anoxide insulating layer without exposure to the air. The deposition ofthe silicon oxide film by a sputtering method can be performed in a raregas (typically, argon) atmosphere, an oxygen atmosphere, or anatmosphere of a rare gas (typically, argon) and oxygen. As a target, asilicon oxide target or a silicon target can be used. For example, withthe use of a silicon target, a silicon oxide film can be formed by asputtering method in an atmosphere of oxygen and a rare gas. For theoxide insulating layer formed in contact with the oxide semiconductorlayer whose resistance is reduced, an inorganic insulating film thatdoes not include impurities such as water, a hydrogen ion, and OH⁻ andblocks entry of these from the outside is preferably used.

In this embodiment, the film deposition is performed by a pulsed DCsputtering method using a columnar polycrystalline, boron-doped silicontarget which has a purity of 6N (with a resistivity of 0.01 Ω·cm), inwhich the distance between the substrate and the target (T-S distance)is 89 mm, the pressure is 0.4 Pa, the direct-current (DC) power sourceis 6 kW, and the atmosphere is oxygen (the oxygen flow rate is 100%).The film thickness thereof is 300 nm.

Next, a second heat treatment is performed in an inert gas atmosphere ora nitrogen gas atmosphere (preferably at a temperature higher than orequal to 200° C. and lower than or equal to 400° C., e.g., higher thanor equal to 250° C. and lower than or equal to 350° C.). For example,the second heat treatment is performed in a nitrogen atmosphere at 250°C. for one hour. Alternatively, an RTA treatment may be performed at ahigh temperature for a short time as in the first heat treatment. By thesecond heat treatment, the oxide insulating layer and the oxidesemiconductor layer overlapping with the oxide insulating layer areheated being in contact with each other. Note that by the second heattreatment, the oxide semiconductor layer 423 whose resistance is reducedby the first heat treatment is in an oxygen excess state and can bechanged into a high-resistance oxide semiconductor layer (an i-typeoxide semiconductor layer).

In this embodiment, the second heat treatment is performed afterformation of the silicon oxide film; however, the timing of the heattreatment is not limited to the timing immediately after formation ofthe silicon oxide film as long as it is after formation of the siliconoxide film.

In the case where the source electrode layer 425 a and the drainelectrode layer 425 b are formed using a heat resistant material, a stepusing conditions of the first heat treatment can be performed at thetiming of the second heat treatment. In that case, a heat treatment maybe performed once after formation of the silicon oxide film.

Then, in a sixth photolithography step, a resist mask is formed and theoxide insulating layer 428 is etched so that a contact hole that reachesthe drain electrode layer 425 b is formed. In addition, a contact holethat reaches the connection electrode 429 is also formed by thisetching.

Next, a transparent conductive film is formed after the resist mask isremoved. The transparent conductive film is formed using an indium oxide(In₂O₃), an alloy of an indium oxide and a tin oxide (In₂O₃—SnO₂,hereinafter abbreviated as ITO), or the like by a sputtering method, avacuum evaporation method, or the like. Such a material is etched with ahydrochloric acid-based solution. Note that since a residue is likely tobe generated in etching ITO in particular, an alloy of an indium oxideand a zinc oxide (In₂O₃—ZnO) may be used to improve etchingprocessability.

Next, in a seventh photolithography step, a resist mask is formed and anunnecessary portion is removed by etching to form a pixel electrodelayer 110.

In the seventh photolithography step, a storage capacitor is formed withthe gate insulating layer 402, the oxide insulating layer 426 b, and theoxide insulating layer 428 in the capacitor portion which are used asdielectrics, the capacitor wiring 421 b, and the pixel electrode layer110.

Further, in the seventh photolithography step, the first terminal 421 cis covered with the resist mask, and a transparent conductive film 128is left in the terminal portion. The transparent conductive film 128serves as an electrode or a wiring connected to an FPC. The transparentconductive film 128 which is formed over the connection electrode 429being directly connected to the first terminal 421 c is a connectionterminal electrode which functions as an input terminal of the gatewiring. Although not illustrated, a connection terminal electrode whichfunctions as an input terminal of the source wiring is also formed atthis time.

FIGS. 4A1 and 4A2 are a cross-sectional view of a gate wiring terminalportion at this stage and a plan view thereof, respectively. FIG. 4A1 isa cross-sectional view taken along line C1-C2 of FIG. 4A2. In FIG. 4A1,a transparent conductive film 415 formed over the oxide insulating layer428 is a connection terminal electrode which functions as an inputterminal. Further, in FIG. 4A1, in the terminal portion, a firstterminal 411 formed using the same material as the gate wiring and theconnection electrode 412 formed using the same material as the sourcewiring overlap with each other with a gate insulating layer 402interposed therebetween and are in direct electrical connection.Furthermore, the connection electrode 412 and the transparent conductivefilm 415 are directly connected to each other through a contact holeformed in the oxide insulating layer 428.

FIGS. 4B 1 and 4B2 are a cross-sectional view of a source wiringterminal portion and a plan view thereof, respectively. FIG. 4B1 is across-sectional view taken along line C3-C4 of FIG. 4B2. In FIG. 4B1, atransparent conductive film 418 formed over the oxide insulating layer428 is a connection terminal electrode which functions as an inputterminal. Further, in FIG. 4B1, in the terminal portion, an electrode416 formed using the same material as the gate wiring is located belowand overlaps with a second terminal 414 electrically connected to thesource wiring, with the gate insulating layer 402 interposedtherebetween. The electrode 416 is not electrically connected to thesecond terminal 414, and a capacitor to prevent noise or staticelectricity can be formed when the electric potential of the electrode416 is set to an electric potential different from that of the secondterminal 414, such as a GND potential or 0 V, or the electrode 416 isset to be in a floating state. The second terminal 414 is electricallyconnected to the transparent conductive film 418 with the oxideinsulating layer 428 interposed therebetween.

A plurality of gate wirings, source wirings, and capacitor wirings areprovided depending on the pixel density. Also in the terminal portion, aplurality of first terminals at the same electric potential as the gatewiring, a plurality of second terminals at the same electric potentialas the source wiring, a plurality of third terminals at the sameelectric potential as the capacitor wiring, and the like are arranged.The number of each of the terminals may be any number, and the number ofthe terminals may be determined by a practitioner as appropriate.

Through these seven photolithography steps, the channel protected thinfilm transistor 470 and the storage capacitor portion can be thuscompleted using the seven photomasks. These transistors and storagecapacitors are arranged in matrix corresponding to respective pixels soas to form the pixel portion, whereby one of the substrates included inan active-matrix display device can be obtained. In this specification,such a substrate is referred to as an active matrix substrate forconvenience.

In the case of manufacturing an active matrix liquid crystal displaydevice, an active matrix substrate and a counter substrate provided witha counter electrode are bonded to each other with a liquid crystal layerinterposed therebetween. Note that a common electrode electricallyconnected to the counter electrode on the counter substrate is providedover the active matrix substrate, and a fourth terminal electricallyconnected to the common electrode is provided in the terminal portion.The fourth terminal is provided so that the common electrode is set to afixed electric potential such as a GND potential or 0 V.

A pixel structure of this embodiment is not limited to the pixelstructure in FIG. 3. For example, a pixel electrode may overlap with agate wiring of an adjacent pixel with the protective insulating film andthe gate insulating layer interposed therebetween to form a storagecapacitor without a capacitor wiring. In this case, the capacitor wiringand the third terminal connected to the capacitor wiring can be omitted.

Further alternatively, the source electrode layer 425 a and the drainelectrode layer 425 b may be over and overlap with the oxide insulatinglayer 456 a which functions as a channel protective layer as illustratedin FIGS. 5A and 5B. In this case, the oxide semiconductor layer is notetched when the source electrode layer 425 a and the drain electrodelayer 425 b are patterned, and therefore, a thinner region is not formedin the oxide semiconductor layer. In other words, the oxidesemiconductor layer has the first region 424 a overlapping with thesource electrode layer 425 a, the second region 424 b overlapping withthe drain electrode layer 425 b, and the fifth region 424 e which is thechannel formation region which have the same thickness.

Further alternatively, a thin film transistor 490 may be employed inwhich the thickness of a region which is amorphous or is formed of amixture of amorphousness/non-crystals and microcrystals in the fifthregion 424 e of the oxide semiconductor layer is less than eachthickness of the third region 424 c and the fourth region 424 d (i.e.,an interface between the crystal region in the fifth region 424 e andthe region which is amorphous or is formed of a mixture ofamorphousness/non-crystals and microcrystals is above the outermostsurfaces of the third region 424 c and the fourth region 424 d) asillustrated in FIG. 22A. The thin film transistor 490 having such astructure can be obtained by adjusting a heating temperature or heatingtime of the first heat treatment so as to make the depth of the crystalregion of the oxide semiconductor layer extremely shallow. By employingthe structure of the thin film transistor 490 which is illustrated inFIG. 22A, off current can be reduced.

The channel length L of the channel protected thin film transistor 490illustrated in FIG. 22A is equal to the width of the oxide insulatinglayer 426 a which is in a direction parallel to a direction in whichcarriers flow. It is to be noted that the sum of the width L₃ in thechannel length direction of the third region and the width L₄ in thechannel length direction of the fourth region is constant in the thinfilm transistor 490 which is illustrated in FIG. 22A, though the widthL₃ in the channel length direction of the third region of the oxidesemiconductor layer is not necessarily equal to the width L₄ in thechannel length direction of the fourth region.

A thin film transistor 430 having a structure in which the first tofifth regions 424 a to 424 e of the oxide semiconductor layer have acrystal region in their superficial portions as illustrated in FIG. 22Bmay be alternatively employed. By employing the structure of the thinfilm transistor 430 which is illustrated in FIG. 22B, on current can beincreased.

Thin film transistors having different structures, which may be selectedfrom the thin film transistors 430, 450, 470, and 490 may be formed overone substrate. Note that in the case where a pixel portion and a drivercircuit are formed over one substrate, excellent switchingcharacteristics are needed for a thin film transistor used for the pixelportion, and high operation speed is preferable for a thin filmtransistor used for the driver circuit; for example, the thin filmtransistor 430 may be placed in the driver circuit portion and the thinfilm transistor 490 may be placed in the pixel portion, as illustratedin FIG. 22C. The thin film transistor 430 placed in the driver circuitportion can increase on current and thus is suitable for applicationswhich need which high current driving capability. The thin filmtransistor 490 placed in the pixel portion can reduce off current andthus can improve contrast when used as a switching element in the pixelportion. Alternatively, as illustrated in FIG. 22D, the thin filmtransistor 450 may be placed in the driver circuit portion and the thinfilm transistor 470 with low off current is preferably placed in thepixel portion. Further alternatively, although not illustrated, the thinfilm transistor 430 and the thin film transistor 470 may be placed inthe driver circuit portion and the pixel portion, respectively, or thethin film transistor 450 and the thin film transistor 490 may be placedin the driver circuit portion and the pixel portion, respectively.

Note that in each of the thin film transistors 430, 450, 470 and 490,the interface between the gate insulating layer 402 and the oxidesemiconductor layer 423 which are in contact with each other isamorphous or is formed of a mixture of amorphousness/non-crystals andmicrocrystals and at least a superficial portion of the oxidesemiconductor layer which is in contact with the oxide insulating layer426 a has a crystal region.

In an active matrix liquid crystal display device, a display pattern isformed on a screen by driving pixel electrodes arranged in matrix.Specifically, voltage is applied between a selected pixel electrode anda counter electrode corresponding to the pixel electrode, so that aliquid crystal layer provided between the pixel electrode and thecounter electrode is optically modulated and this optical modulation isrecognized as a display pattern by an observer.

In displaying moving images of a liquid crystal display device, there isa problem in that a long response time of liquid crystal moleculesthemselves causes afterimages or blurring of moving images. In order toimprove the moving-image characteristics of a liquid crystal displaydevice, a driving method called black insertion is employed in whichblack is displayed on the whole screen every other frame period.

Further, there is another driving technique which is so-calleddouble-frame rate driving. In the double-frame rate driving, a verticalsynchronizing frequency is set 1.5 times or more, preferably, 2 times ormore as high as a usual vertical synchronizing frequency, whereby theresponse speed is increased, and the grayscale to be written is selectedfor every plural fields in each frame which have been obtained bydividing.

Further alternatively, in order to improve the moving-imagecharacteristics of a liquid crystal display device, a driving method maybe employed, in which a plurality of LEDs (light-emitting diodes) or aplurality of EL light sources are used to form a plane light source as abacklight, and each light source of the plane light source isindependently driven in a pulsed manner in one frame period. Three ormore kinds of LEDs may be used or an LED that emits white light may beused. Since a plurality of LEDs can be controlled independently, thelight emission timing of the LEDs can be synchronized with the timing atwhich a liquid crystal layer is optically modulated. According to thisdriving method, LEDs can be partly turned off; therefore, an effect ofreducing power consumption can be obtained particularly in the case ofdisplaying an image having a large black display region occupied in onescreen.

By combining these driving methods, the display characteristics of aliquid crystal display device, such as moving-image characteristics, canbe improved as compared with those of conventional liquid crystaldisplay devices.

The n-channel transistor obtained in this embodiment includes anIn—Ga—Zn—O-based film in a channel formation region and has good dynamiccharacteristics. Thus, these driving methods can be applied incombination to the transistor of this embodiment.

In manufacturing a light-emitting display device, one electrode (alsoreferred to as a cathode) of an organic light-emitting element is set toa low power supply potential such as a GND potential or 0 V; thus, aterminal portion is provided with a fourth terminal for setting thecathode to a low power supply potential such as a GND potential or 0 V.Also in manufacturing a light-emitting display device, a power supplyline is provided in addition to a source wiring and a gate wiring.Accordingly, the terminal portion is provided with a fifth terminalelectrically connected to the power supply line.

Through the above steps, a highly reliable thin film transistor havingfavorable electrical characteristics and a display device including thethin film transistor can be provided.

The thin film transistor described in this embodiment is a thin filmtransistor using an oxide semiconductor layer. At least the superficialportion of the oxide semiconductor layer in the channel formation regionhas a crystal region and the rest portion of the oxide semiconductorlayer can be amorphous or be formed of a mixture ofamorphousness/non-crystals and microcrystals, which enables the thinfilm transistor to suppress generation of a parasitic channel.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 2

In this embodiment, an example of a manufacturing process of a displaydevice different from that in Embodiment 1 is described with referenceto FIGS. 6A to 6E. Note that in this embodiment, the same portions asthose in Embodiment 1 and portions having functions similar to those inEmbodiment 1 can be treated as in Embodiment 1, and the same or similarsteps as or to those in Embodiment 1 can be performed as inEmbodiment 1. Thus, repeated description is omitted.

First, over the substrate 400 having an insulating surface, a conductivefilm for forming the gate wiring including the gate electrode layer 421a, the capacitor wiring 421 b, and the first terminal 421 c is formed bya sputtering method or a vacuum evaporation method. Next, after theformation of the conductive film over the entire surface of thesubstrate 400, in a first photolithography step, a resist mask is formedand an unnecessary portion is removed by etching to form wirings and anelectrode (the gate wiring including the gate electrode layer 421 a, thecapacitor wiring 421 b, and the first terminal 421 c).

Next, over the gate electrode layer 421 a, the capacitor wiring 421 b,and the first terminal 421 c, the gate insulating layer 402 is formed;then, over the gate insulating layer 402, an oxide semiconductor film103 is formed to a thickness greater than or equal to 5 nm and less thanor equal to 200 nm, preferably, greater than or equal to 10 nm and lessthan or equal to 40 nm. Note that the above steps can be performed as inEmbodiment 1.

Next, over the oxide semiconductor film 103, an oxide insulating film105 is formed by a sputtering method; then, in a second photolithographystep, a resist mask is formed and selective etching is performed thereonso that a contact hole reaching the first terminal 421 c is formed (seeFIG. 6A). The oxide insulating film 105 can be formed in a mannersimilar to that of the oxide insulating film to be the oxide insulatinglayer 426 a described in Embodiment 1.

Then, the oxide semiconductor film 103 is subjected to dehydration ordehydrogenation. A first heat treatment for the dehydration ordehydrogenation can be performed by a rapid thermal annealing (RTA)treatment with the use of a high-temperature gas (an inert gas such asnitrogen or a rare gas) or light at a temperature higher than or equalto 500° C. and lower than or equal to 750° C. (or a temperature lowerthan or equal to the strain point of a glass substrate) forapproximately greater than or equal to one minute and less than or equalto ten minutes, preferably at 650° C. for approximately greater than orequal to three minutes and less than or equal to six minutes. By an RTAtreatment, dehydration or dehydrogenation can be performed in a shorttime; therefore, a treatment can be performed even at a temperaturehigher than the strain point of a glass substrate. Note that the timingof the heat treatment is not limited to this timing and may be performedplural times, for example, before and after a photolithography step or adeposition step.

Here, the superficial portion of the oxide semiconductor film 103 iscrystallized by the first heat treatment and thus comes to have thedense crystal region 106 including nanocrystals. The rest portion of theoxide semiconductor film 103 comes to be amorphous or be formed of amixture of amorphousness/non-crystals and microcrystals, where anamorphous region is dotted with microcrystals. Note that the crystalregion 106 is part of the oxide semiconductor film 103 and hereinafter,the “oxide semiconductor film 103” includes the crystal region 106.

In addition, when the temperature is lowered from the heatingtemperature T at which the oxide semiconductor film is subjected to thedehydration or dehydrogenation, it is important to prevent entry ofwater or hydrogen by using the same furnace that has been used for thedehydration or dehydrogenation, in such a manner that the oxidesemiconductor layer is not exposed to the air. In addition, the gasatmosphere in which the temperature is lowered from the heatingtemperature T may be switched to a gas atmosphere which is differentfrom the gas atmosphere in which the temperature is raised to theheating temperature T. For example, cooling is performed in the furnacewhere the heat treatment for dehydration or dehydrogenation is performedwhile the furnace is filled with a high-purity oxygen gas, a high-purityN₂O gas, or ultra-dry air (having a dew point lower than or equal to−40° C., preferably lower than or equal to −60° C.) without exposure tothe air.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the atmosphere.Alternatively, the purity of an inert gas which is introduced into aheat treatment apparatus is preferably greater than or equal to 6N(99.9999%), more preferably greater than or equal to 7N (99.99999%)(that is, the impurity concentration is less than or equal to 1 ppm,preferably less than or equal to 0.1 ppm).

By the first heat treatment, the oxide semiconductor film 103 is changedto an oxygen-deficient type and the resistance thereof is reduced. Afterthe first heat treatment, the carrier concentration is higher than thatof the oxide semiconductor film just after the film deposition, so thatthe oxide semiconductor film has a carrier concentration of, preferably,greater than or equal to 1×10¹⁸/cm³.

Then, in a third photolithography step, a resist mask is formed and theoxide insulating layers 426 a, 426 b, 426 c, and 426 d are formed byselective etching. After that, the resist mask is removed (see FIG. 6B).Here, the oxide insulating layer 426 a functions as a channel protectivelayer of a thin film transistor. Further, in the oxide semiconductorfilm 103, a region overlapping with the oxide insulating layer 426 a isa region to be a channel formation region in a later step.

Then, a conductive film formed using a metal material is formed over theoxide semiconductor film 103 and the oxide insulating layers 426 a, 426b, 426 c, and 426 d by a sputtering method, a vacuum evaporation method,or the like. As a material of the conductive film, a material similar tothat of the gate electrode layer 421 a can be used.

In this embodiment, a conductive film in which first to third conductivefilms are stacked is formed. For example, the first conductive film andthe third conductive film are formed using titanium that is aheat-resistant conductive material, and the second conductive film isformed using an aluminum alloy containing neodymium. Such a structurecan reduce generation of hillock and utilize a low resistance propertyof aluminum. Although a three-layer structure of the first to thirdconductive films is used in this embodiment, one embodiment of thepresent invention is not limited to this. A single-layer structure, atwo-layer structure, or a stacked-layer structure of four or more layersmay be employed. A structure including two layers or four or more layersmay be employed. For example, a single-layer structure of a titaniumfilm or a single-layer structure of an aluminum film containing siliconmay be employed.

Note that at the time of deposition of the conductive film on and incontact with the oxide semiconductor layer, whose superficial portionhas the dense crystal region 106 including nanocrystals, damage on thecrystal region due to the deposition or heat by the deposition makes thecrystal region 106 of the semiconductor layer amorphous in some cases.However, in the manufacturing method of a thin film transistor which isdescribed in this embodiment, the oxide insulating layer 426 afunctioning as a channel protective layer is provided in contact withthe region to be the channel formation region of the oxide semiconductorlayer, whereby the superficial portion of the oxide semiconductor layer423 at least in the channel formation region can have the crystal region106.

Next, in a fourth photolithography step, resist masks 480 a and 480 bare formed and unnecessary portions are removed by etching, so that theconductive layer 425 and the connection electrode 429 are formed (seeFIG. 6C). Wet etching or dry etching is employed as an etching method atthis time. For example, when the first conductive film and the thirdconductive film are formed using titanium and the second conductive filmis formed using an aluminum alloy containing neodymium, wet etching canbe performed using a hydrogen peroxide solution or heated hydrochloricacid as an etchant.

In addition, by this fourth photolithography step, the connectionelectrode 429 is directly connected to the first terminal 421 c of theterminal portion through a contact hole formed in the gate insulatinglayer. Although not illustrated, a source or drain wiring and a gateelectrode of a thin film transistor of a driver circuit are directlyconnected to each other through the same steps as the above steps.

The resist masks 480 a and 480 b in this embodiment can also be referredto as a resist mask having a recessed portion or a projected portion. Inother words, the resist masks 480 a and 480 b can be referred to as aresist mask having a plurality of regions (here, two regions) withdifferent thicknesses. In the resist masks 480 a and 480 b, a regionwith a larger thickness is referred to as a projected portion and aregion with a smaller thickness is referred to as a recessed portion.

In each of the resist masks 480 a and 480 b, the projected portion isformed corresponding to a region of the conductive film, which is to bea source electrode layer or a drain electrode layer later, and therecessed portion is formed corresponding to a peripheral portion of anisland-shaped oxide semiconductor layer which is described later.

The resist masks described in this embodiment can be formed using amulti-tone mask. The multi-tone mask is a mask capable of light exposurewith multi-level light intensity, and typically, light exposure isperformed with three levels of light intensity to provide an exposedregion, a half-exposed region, and an unexposed region. With the use ofa multi-tone mask, a resist mask with plural thicknesses (typically, twokinds of thicknesses) can be formed by one light exposure anddevelopment step. Thus, by using the multi-tone mask, the number ofphotomasks can be reduced.

By light exposure using the multi-tone mask and development, the resistmasks 480 a and 480 b each of which has regions with differentthicknesses can be formed. Note that without limitation thereto, theresist masks 480 a and 480 b may be formed without the multi-tone mask.

After the conductive layer 425 and the connection electrode 429 areformed using the resist masks 480 a and 480 b, the resist masks 480 aand 480 b are reduced (downsized) to form resist masks 482 a, 482 b, and482 c. In order to reduce (downsize) the resist masks 480 a and 480 b,ashing using oxygen plasma or the like may be performed. By the reducing(downsizing) the resist masks, a recessed portion of the resist mask 480a is disappeared and divided into the resist masks 482 a and 482 b.Further, the conductive layer 425 in a region between the resist mask482 a and 482 b is exposed (not shown).

Next, using the resist masks 482 a, 482 b, and 482 c, an exposed part ofthe conductive layer 425 is etched and the connection electrode 429 ispartly etched. Accordingly, a source electrode 425 a, a drain electrode425 b and an island-shaped oxide semiconductor layer 423 are formed (seeFIG. 6D).

By this etching step, the oxide semiconductor film 103 is partly etchedso that the third region 424 c between the source electrode layer 425 aand the oxide insulating layer 426 a and the fourth region 424 d betweenthe drain electrode layer 425 b and the oxide insulating layer 426 aeach have a thickness less than each of the first region 424 aoverlapping with the source electrode layer 425 a, and the second region424 b overlapping with the drain electrode layer 425 b, and the fifthregion 424 e overlapping with the oxide insulating layer 426 a. Notethat the fifth region 424 e of the oxide semiconductor layer 423 isprotected by the oxide insulating layer 426 a so as not to be etched andthus the superficial portion has a dense crystal region includingnanocrystals at least in the channel formation region. In the channelformation region, the superficial portion of the oxide semiconductorlayer is on the back channel side and the crystal region can suppressgeneration of a parasitic channel.

Each thickness of the first region 424 a and the second region 424 b isequal to that of the fifth region 424 e that is the channel formationregion.

Next, the oxide insulating layer 428 which covers a thin film transistor410 is formed (see FIG. 6E). As the oxide insulating layer 428, an oxideinsulating layer formed using a silicon oxide film, a silicon oxynitridefilm, an aluminum oxide film, an aluminum oxynitride film, or a tantalumoxide film which is formed by a sputtering method or the like.

Next, a second heat treatment is performed in an inert gas atmosphere ora nitrogen gas atmosphere (preferably at a temperature higher than orequal to 200° C. and lower than or equal to 400° C., e.g., higher thanor equal to 250° C. and lower than or equal to 350° C.). For example,the second heat treatment is performed in a nitrogen atmosphere at 250°C. for one hour. Alternatively, an RTA treatment may be performed at ahigh temperature for a short time as in the first heat treatment. By thesecond heat treatment, the oxide insulating layer and the oxidesemiconductor layer overlapping with the oxide insulating layer areheated being in contact with each other. Note that by the second heattreatment, the oxide semiconductor layer 423 whose resistance is reducedby the first heat treatment is in an oxygen excess state and can bechanged into a high-resistance oxide semiconductor layer (an i-typeoxide semiconductor layer).

In this embodiment, the second heat treatment is performed afterformation of the silicon oxide film; however, the timing of the heattreatment is not limited to the timing immediately after formation ofthe silicon oxide film as long as it is after formation of the siliconoxide film.

In the case where the source electrode layer 425 a and the drainelectrode layer 425 b are formed using a heat resistant material, a stepusing conditions of the first heat treatment can be performed at thetiming of the second heat treatment. In that case, a heat treatment maybe performed once after formation of the silicon oxide film.

Note that a protective insulating layer may be formed over the oxideinsulating layer 428. As the protective insulating layer, a siliconnitride film can be formed by an RF sputtering method, for example. Theprotective insulating layer is formed using an inorganic insulating filmwhich does not contain impurities such as water, hydrogen ions, and OH⁻and prevents entry of these from the outside. A silicon nitride film, analuminum nitride film, a silicon nitride oxide film, an aluminumoxynitride film, or the like is used. Note also that the protectiveinsulating layer can be formed successively following the oxideinsulating layer 428.

Then, in a fifth photolithography step, a resist mask is formed and theoxide insulating layer 428 is etched so that a contact hole that reachesthe drain electrode layer 425 b is formed. In addition, a contact holethat reaches the connection electrode 429 is also formed by thisetching.

Next, a transparent conductive film is formed after the resist mask isremoved. The transparent conductive film is formed using an indium oxide(In₂O₃), an alloy of an indium oxide and a tin oxide (In₂O₃—SnO₂,hereinafter abbreviated as ITO), or the like by a sputtering method, avacuum evaporation method, or the like. Such a material is etched with ahydrochloric acid-based solution. Note that since a residue is likely tobe generated in etching ITO in particular, an alloy of an indium oxideand a zinc oxide (In₂O₃—ZnO) may be used to improve etchingprocessability.

Next, in a sixth photolithography step, a resist mask is formed and anunnecessary portion is removed by etching to form the pixel electrodelayer 110.

In the sixth photolithography step, a storage capacitor is formed withthe gate insulating layer 402, the oxide semiconductor layer, the oxideinsulating layer 426 b, and the oxide insulating layer 428 in thecapacitor portion which are used as dielectrics, the capacitor wiring421 b, and the pixel electrode layer 110.

Further, in the sixth photolithography step, the first terminal 421 c iscovered with the resist mask, and a transparent conductive film 128 isleft in the terminal portion. The transparent conductive film 128 servesas an electrode or a wiring connected to an FPC. The transparentconductive film 128 which is formed over the connection electrode 429being directly connected to the first terminal 421 c is a connectionterminal electrode which functions as an input terminal of the gatewiring. Although not illustrated, a connection terminal electrode whichfunctions as an input terminal of the source wiring is also formed atthis time.

Through these six photolithography steps, the channel protected thinfilm transistor 410 and the storage capacitor portion can be thuscompleted using the six photomasks.

The thin film transistor described in this embodiment is a thin filmtransistor using an oxide semiconductor layer. A superficial portion ofthe oxide semiconductor layer in the channel formation region has acrystal region and the rest portion of the oxide semiconductor layer canbe amorphous or be formed of a mixture of amorphousness/non-crystals andmicrocrystals. With a thin film transistor having this structure,generation of a parasitic channel can be suppressed; accordingly, ahighly reliable thin film transistor and a display device which havefavorable electrical characteristics can be manufactured.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 3

In this embodiment, an example is described below in which at least partof a driver circuit and a thin film transistor to be placed in a pixelportion are formed over one substrate.

The thin film transistor placed in the pixel portion is formed accordingto Embodiment 1 or 2. Further, the thin film transistor described inEmbodiment 1 or 2 is an n-channel TFT. Thus, some of driver circuitsthat can be constituted by n-channel TFTs among the driver circuits areformed over the same substrate as that for the thin film transistor inthe pixel portion.

FIG. 7A illustrates an example of a block diagram of an active matrixdisplay device. A pixel portion 5301, a first scan line driver circuit5302, a second scan line driver circuit 5303, and a signal line drivercircuit 5304 are provided over a substrate 5300 in the display device.In the pixel portion 5301, a plurality of signal lines extending fromthe signal line driver circuit 5304 is provided, and a plurality of scanlines extending from the first scan line driver circuit 5302 and thesecond scan line driver circuit 5303 is provided. Note that in crossregions of the scan lines and the signal lines, pixels each having adisplay element are arranged in matrix. Further, the substrate 5300 ofthe display device is connected to a timing control circuit 5305 (alsoreferred to as a controller or a control IC) through a connectionportion of a flexible printed circuit (FPC) or the like.

In FIG. 7A, the first scan line driver circuit 5302, the second scanline driver circuit 5303, and the signal line driver circuit 5304 areformed over the same substrate 5300 as the pixel portion 5301.Accordingly, the number of parts such as driver circuits providedoutside is reduced, so that cost can decrease. Moreover, the number ofconnections in the connection portion in the case where wirings areextended from a driver circuit provided outside the substrate 5300 canbe reduced, and the reliability or yield can be improved.

Note that the timing control circuit 5305 supplies a start signal forthe first scan line driver circuit (GSP1) and a clock signal for thescan line driver circuit (GCK1) to the first scan line driver circuit5302, as an example. In addition, the timing control circuit 5305supplies, for example, a start signal for the second scan line drivercircuit (GSP2) (also referred to as a start pulse) and a clock signalfor the scan line driver circuit (GCK2) to the second scan line drivercircuit 5303. A start signal for the signal line driver circuit (SSP), aclock signal for the signal line driver circuit (SCK), data for a videosignal (DATA) (also simply referred to as a video signal), and a latchsignal (LAT) are supplied to the signal line driver circuit 5304. Notethat each clock signal may be a plurality of clock signals withdifferent phases, or may be supplied with an inverted clock signal(CKB). Note that either the first scan line driver circuit 5302 or thesecond scan line driver circuit 5303 can be omitted.

In FIG. 7B, a circuit with a low drive frequency (e.g., the first scanline driver circuit 5302 and the second scan line driver circuit 5303)is formed over the same substrate 5300 as the pixel portion 5301, andthe signal line driver circuit 5304 is formed over another substratewhich is different from the substrate provided with the pixel portion5301. This structure enables a driver circuit formed over the substrate5300 using a thin film transistor having low field effect mobility,compared with a transistor formed using a single crystal semiconductor.Accordingly, increase in the size of the display device, a reduction inthe number of steps, a reduction in cost, improvement in yield, or thelike can be achieved.

The thin film transistor described in Embodiment 1 or 2 is an n-channelTFT. In FIGS. 8A and 8B, an example of a structure and operation of asignal line driver circuit formed using an n-channel TFT is described.

The signal line driver circuit includes a shift register 5601 and aswitching circuit 5602. The switching circuit 5602 includes a pluralityof switching circuits 5602_1 to 5602_N (N is a natural number). Theswitching circuits 5602_1 to 5602_N each include a plurality of thinfilm transistors 5603_1 to 5603 _(—) k (k is a natural number). Anexample in which the thin film transistors 5603_1 to 5603 _(—) k aren-channel TFTs is described.

A connection relation of the signal line driver circuit is described byusing the switching circuit 5602_1 as an example. First terminals of thethin film transistors 5603_1 to 5603 _(—) k are connected to wirings5604_1 to 5604_κ, respectively. Second terminals of the thin filmtransistors 5603_1 to 5603 _(—) k are connected to signal lines S1 toSk, respectively. Gates of the thin film transistors 5603_1 to 5603 _(—)k are connected to a wiring 5605_1.

The shift register 5601 has a function of sequentially outputting Hlevel signals (also referred to as an H signal or a high power electricpotential level) to the wirings 5605_1 to 5605_N, and a function ofsequentially selecting the switching circuits 5602_1 to 5602_N.

The switching circuit 5602_1 has a function of controlling conductionstates between the wirings 5604_1 to 5604 _(—) k and the signal lines S1to Sk (conduction between the first terminal and the second terminal),that is, a function of controlling whether the electric potentials ofthe wirings 5604_1 to 5604 _(—) k are supplied or not to the signallines S1 to Sk. In this manner, the switching circuit 5602_1 has afunction of a selector. The thin film transistors 5603_1 to 5603 _(—) khave functions of controlling conduction states between the wirings5604_1 to 5604 _(—) k and the signal lines S1 to Sk, that is, functionsof supplying electric potentials of the wirings 5604_1 to 5604 _(—) k tothe signal lines S1 to Sk, respectively. In this manner, each of thethin film transistors 5603_1 to 5603 _(—) k functions as a switch.

Note that the data for a video signal (DATA) is input to the wirings5604_1 to 5604 _(—) k. The data for a video signal (DATA) is an analogsignal corresponding to image data or an image signal in many cases.

Next, operation of the signal line driver circuit illustrated in FIG. 8Ais described with reference to a timing chart in FIG. 8B. In FIG. 8B, anexample of signals Sout_1 to Sout_N and signals Vdata_1 to Vdata_k isillustrated. The signals Sout_1 to Sout_N are examples of output signalsof the shift register 5601, and the signals Vdata_1 to Vdata_k areexamples of signals which are input to the wirings 5604_1 to 5604 _(—)k, respectively. Note that one operation period of the signal linedriver circuit corresponds to one gate selection period in a displaydevice. For example, one gate selection period is divided into periodsT1 to TN. The periods T1 to TN are periods for writing the data for avideo signal (DATA) to pixels in a selected row.

In the periods T1 to TN, the shift register 5601 sequentially outputs Hlevel signals to the wirings 5605_1 to 5605_N. For example, in theperiod T1, the shift register 5601 outputs a high level signal to thewiring 5605_1. Then, the thin film transistors 5603_1 to 5603 _(—) k areturned on, so that the wirings 5604_1 to 5604 _(—) k and the signallines S1 to Sk are brought into conduction. In this case, Data (S1) toData (Sk) are input to the wirings 5604_1 to 5604 _(—) k, respectively.The Data (Si) to Data (Sk) are input to pixels in a selected row in afirst to k-th columns through the thin film transistors 5603_1 to 5603_(—) k, respectively. Thus, in the periods T1 to TN, the data for avideo signal (DATA) is sequentially written to the pixels in theselected row by k columns.

By writing the data for a video signal (DATA) to pixels by a pluralityof columns, the number of the data for a video signal (DATA) or thenumber of wirings can be reduced. Accordingly, the number of connectionsto external circuits can be reduced. Further, by writing a video signalto pixels of a plurality of columns each time, write time can beextended, and shortage of writing of a video signal can be prevented.

Note that for the shift register 5601 and the switching circuit 5602, acircuit which is formed using the thin film transistor described inEmbodiment 1 or 2 can be used. In that case, all the transistorsincluded in the shift register 5601 can be only n-channel transistors oronly p-channel transistors.

The structure of a scan line driver circuit will be described. The scanline driver circuit includes a shift register. Additionally, the scanline driver circuit may include a level shifter, a buffer, or the likein some cases. In the scan line driver circuit, when the clock signal(CLK) and the start pulse signal (SP) are input to the shift register, aselection signal is generated. The generated selection signal isbuffered and amplified by the buffer, and the resulting signal issupplied to a corresponding scan line. Gate electrodes of transistors inpixels of one line are connected to the scan line. Since the transistorsin the pixels of one line have to be turned on all at once, a bufferwhich can feed a large amount of current is used.

One mode of a shift register which is used for part of a scan linedriver circuit and/or a signal line driver circuit is described withreference to FIGS. 9A to 9C and FIGS. 10A and 10B.

The shift register includes a first to Nth pulse output circuits 10_1 to10_N (N is a natural number greater than or equal to 3) (see FIG. 9A).In the shift register illustrated in FIG. 9A, a first clock signal CK1,a second clock signal CK2, a third clock signal CK3, and a fourth clocksignal CK4 are supplied from a first wiring 11, a second wiring 12, athird wiring 13, and a fourth wiring 14, respectively, to the first toNth pulse output circuits 10_1 to 10_N. A start pulse SP1 (a first startpulse) is input from a fifth wiring 15 to the first pulse output circuit10_1. To the nth pulse output circuit 10 _(—) n (n is a natural numbergreater than or equal to 2 and less than or equal to N) in the second orlater stage, a signal from the pulse output circuit in the precedingstage (such a signal is referred to as a preceding-stage signalOUT(n−1)) is input. A signal from the third pulse output circuit 10_3 inthe stage that is two stages after the first pulse output circuit 10_1is also input to the first pulse output circuit 10_1. In a similarmanner, a signal from the (n+2)th pulse output circuit 10_(n+2) in thestage that is two stages after the nth pulse output circuit 10 _(—) n(such a signal is referred to as a later-stage signal OUT(n+2)) is inputto the nth pulse output circuit 10 _(—) n in the second or later stage.Thus, the pulse output circuits in the respective stages output firstoutput signals (OUT(1)(SR) to OUT (N)(SR)) to be input to the pulseoutput circuits in the respective subsequent stages and/or the pulseoutput circuits in the stages that are two stages before the respectivepulse output circuits and second output signals (OUT(1) to OUT (N)) forelectrical connection to other wirings or the like. Note that since thesubsequent-stage signal OUT(n+2) is not input to the last two stages ofthe shift register as illustrated in FIG. 9A, a second start pulse SP2from the sixth wiring 16 and a third start pulse SP3 from the seventhwiring 17 may be input to the stage before the last stage and the laststage, respectively, for example. Alternatively, a signal generated inthe shift register may be additionally input. For example, a structuremay be employed in which a (n+1)th pulse output circuit 10(n+1) and a(n+2)th pulse output circuit 10(n+2) which do not affect pulse output tothe pixel portion (such circuits are also referred to as dummy stages)are provided so that a signal serving as a second start pulse (SP2) anda signal serving as a third start pulse (SP3) are generated from thedummy stages.

Note that a clock signal (CK) is a signal which alternates between an Hlevel signal and an L level signal (also referred to as an L signal or alow power supply potential level) at a regular interval. Here, the firstto fourth clock signals (CK1) to (CK4) are sequentially delayed by aquarter of a cycle. In this embodiment, by using the first to fourthclock signals (CK1) to (CK4), control or the like of driving of a pulseoutput circuit is performed. Although the clock signal is used as a GCKor an SCK in accordance with a driver circuit to which the clock signalis input, the clock signal is described as a CK here.

A first input terminal 21, a second input terminal 22, and a third inputterminal 23 are electrically connected to any of the first to fourthwirings 11 to 14. For example, in FIG. 9A, the first input terminal 21of the first pulse output circuit 10_1 is electrically connected to thefirst wiring 11, the second input terminal 22 of the first pulse outputcircuit 10_1 is electrically connected to the second wiring 12, and thethird input terminal 23 of the first pulse output circuit 10_1 iselectrically connected to the third wiring 13. In addition, the firstinput terminal 21 of the second pulse output circuit 10_2 iselectrically connected to the second wiring 12, the second inputterminal 22 of the second pulse output circuit 10_2 is electricallyconnected to the third wiring 13, and the third input terminal 23 of thesecond pulse output circuit 10_2 is electrically connected to the fourthwiring 14.

Each of the first to Nth pulse output circuits 10_1 to 10_N includes thefirst input terminal 21, the second input terminal 22, the third inputterminal 23, a fourth input terminal 24, a fifth input terminal 25, afirst output terminal 26, and a second output terminal 27 (see FIG. 9B).In the first pulse output circuit 10_1, the first clock signal CK1 isinput to the first input terminal 21, the second clock signal CK2 isinput to the second input terminal 22, the third clock signal CK3 isinput to the third input terminal 23, the start pulse is input to thefourth input terminal 24, a subsequent stage signal OUT (3) is input tothe fifth input terminal 25, a first output signal OUT (1) (SR) isoutput from the first output terminal 26, and a second output signal OUT(1) is output from the second output terminal 27.

Next, an example of a specific circuit structure of the pulse outputcircuit illustrated in FIG. 9B is described with reference to FIG. 9C.

The pulse output circuit illustrated in FIG. 9C includes first toeleventh transistors 31 to 41. In addition to the first to fifth inputterminals 21 to 25, the first output terminal 26, and the second outputterminal 27, signals or power supply potentials are supplied to thefirst to eleventh transistors 31 to 41 from a power supply line 51 towhich a first high power supply potential VDD is supplied, a powersupply line 52 to which a second high power supply potential VCC issupplied, and a power supply line 53 to which a low power supplypotential VSS is supplied. Here, the magnitude relation among powersupply potentials of the power supply lines illustrated in FIG. 9C isset as follows: the first power supply potential VDD is higher than orequal to the second power supply potential VCC, and the second powersupply potential VCC is higher than the third power supply potentialVSS. Although the first to fourth clock signals (CK1) to (CK4) aresignals which alternate between an H level signal and an L level signalat a regular interval, an electric potential is VDD when the clocksignal is at an H level, and an electric potential is VSS when the clocksignal is at an L level. Note that the electric potential VDD of thepower supply line 51 is higher than the electric potential VCC of thepower supply line 52, so that there is no effect on an operation, theelectric potential applied to a gate electrode of a transistor can below, a shift of the threshold voltage of the transistor can be reduced,and deterioration can be suppressed.

In FIG. 9C, a first terminal of the first transistor 31 is electricallyconnected to the power supply line 51, a second terminal of the firsttransistor 31 is electrically connected to a first terminal of the ninthtransistor 39, and a gate electrode of the first transistor 31 iselectrically connected to the fourth input terminal 24. A first terminalof the second transistor 32 is electrically connected to the powersupply line 53, a second terminal of the second transistor 32 iselectrically connected to the first terminal of the ninth transistor 39,and a gate electrode of the second transistor 32 is electricallyconnected to a gate electrode of the fourth transistor 34. A firstterminal of the third transistor 33 is electrically connected to thefirst input terminal 21, and a second terminal of the third transistor33 is electrically connected to the first output terminal 26. A firstterminal of the fourth transistor 34 is electrically connected to thepower supply line 53, and a second terminal of the fourth transistor 34is electrically connected to the first output terminal 26. A firstterminal of the fifth transistor 35 is electrically connected to thepower supply line 53, a second terminal of the fifth transistor 35 iselectrically connected to the gate electrode of the second transistor 32and the gate electrode of the fourth transistor 34, and a gate electrodeof the fifth transistor 35 is electrically connected to the fourth inputterminal 24. A first terminal of the sixth transistor 36 is electricallyconnected to the power supply line 52, a second terminal of the sixthtransistor 36 is electrically connected to the gate electrode of thesecond transistor 32 and the gate electrode of the fourth transistor 34,and a gate electrode of the sixth transistor 36 is electricallyconnected to the fifth input terminal 25. A first terminal of theseventh transistor 37 is electrically connected to the power supply line52, a second terminal of the seventh transistor 37 is electricallyconnected to a second terminal of the eighth transistor 38, and a gateelectrode of the seventh transistor 37 is electrically connected to thethird input terminal 23. A first terminal of the eighth transistor 38 iselectrically connected to the gate electrode of the second transistor 32and the gate electrode of the fourth transistor 34, and a gate electrodeof the eighth transistor 38 is electrically connected to the secondinput terminal 22. A first terminal of the ninth transistor 39 iselectrically connected to the second terminal of the first transistor 31and the second terminal of the second transistor 32, a second terminalof the ninth transistor 39 is electrically connected to the gateelectrode of the third transistor 33 and a gate electrode of the tenthtransistor 40, and a gate electrode of the ninth transistor 39 iselectrically connected to the power supply line 52. A first terminal ofthe tenth transistor 40 is electrically connected to the first inputterminal 21, a second terminal of the tenth transistor 40 iselectrically connected to the second output terminal 27, and a gateelectrode of the tenth transistor 40 is electrically connected to thesecond terminal of the ninth transistor 39. A first terminal of theeleventh transistor 41 is electrically connected to the power supplyline 53, a second terminal of the eleventh transistor 41 is electricallyconnected to the second output terminal 27, and a gate electrode of theeleventh transistor 41 is electrically connected to the gate electrodeof the second transistor 32 and the gate electrode of the fourthtransistor 34.

In FIG. 9C, a connection portion of the gate electrode of the thirdtransistor 33, the gate electrode of the tenth transistor 40, and thesecond terminal of the ninth transistor 39 is a node A. A connectionportion of the gate electrode of the second transistor 32, the gateelectrode of the fourth transistor 34, the second terminal of the fifthtransistor 35, the second terminal of the sixth transistor 36, the firstterminal of the eighth transistor 38, and the gate electrode of theeleventh transistor 41 is a node B (see FIG. 10A).

Note that a thin film transistor is an element having at least threeterminals of a gate, a drain, and a source. The thin film transistor hasa channel region between a drain region and a source region, and currentcan flow through the drain region, the channel region, and the sourceregion. Here, since the source and the drain of the thin film transistormay change depending on the structure, the operating condition, and thelike of the thin film transistor, it is difficult to define which is asource or a drain. Therefore, a region functioning as a source or adrain is not called the source or the drain in some cases. In such acase, for example, one of the source and the drain may be referred to asa first terminal and the other thereof may be referred to as a secondterminal.

Here, a timing chart of a shift register in which a plurality of pulseoutput circuits illustrated in FIG. 10A is provided is illustrated inFIG. 10B. Note that in FIG. 10B, when the shift register is a scan linedriver circuit, a period 61 is a vertical retrace period and a period 62is a gate selection period.

Note that as illustrated in FIG. 10A, when the ninth transistor 39having the gate to which the second power supply potential VCC isapplied is provided, there are the following advantages before or afterthe bootstrap operation.

Without the ninth transistor 39 whose gate electrode is supplied withthe second power supply potential VCC, when an electric potential of thenode A is raised by bootstrap operation, an electric potential of asource which is the second terminal of the first transistor 31 increasesto a value higher than the first power supply potential VDD. Then, thesource of the first transistor 31 is switched to the first terminalside, that is, the power supply line 51 side. Therefore, in the firsttransistor 31, a large amount of bias voltage is applied and thus greatstress is applied between a gate and a source and between the gate and adrain, which can cause deterioration in the transistor. When the ninthtransistor 39 is provided whose gate electrode is supplied with thesecond power supply potential VCC, an electric potential of the node Ais raised by bootstrap operation, but at the same time, an increase inan electric potential of the second terminal of the first transistor 31can be prevented. In other words, with the ninth transistor 39, negativebias voltage applied between a gate and a source of the first transistor31 can be reduced. Accordingly, with a circuit structure in thisembodiment, negative bias voltage applied between a gate and a source ofthe first transistor 31 can be reduced, so that deterioration in thefirst transistor 31, which is due to stress, can be restrained.

Note that the ninth transistor 39 may be provided in any places wherethe ninth transistor 39 is connected between the second terminal of thefirst transistor 31 and the gate of the third transistor 33 through thefirst terminal and the second terminal. When a shift register includes aplurality of pulse output circuits in this embodiment, the ninthtransistor 39 may be omitted in a signal line driver circuit which has alarger number of stages than a scan line driver circuit, and there is anadvantage of decreasing the number of transistors.

Note that when oxide semiconductors are used for semiconductor layersfor the first to the eleventh transistors 31 to 41, the off-statecurrent of the thin film transistors can be reduced, the on-statecurrent and the field effect mobility can be increased, and the degreeof deterioration can be reduced, whereby malfunction of a circuit candecrease. Compared with a transistor formed using an oxide semiconductorand a transistor formed using amorphous silicon, the degree ofdeterioration of the transistor due to the application of a highelectric potential to the gate electrode is low. Therefore, similaroperation can be obtained even when the first power supply potential VDDis supplied to the power supply line which supplies the second powersupply potential VCC, and the number of power supply lines which are ledbetween circuits can decrease; therefore, the size of the circuit can bereduced.

Note that a similar function is obtained even when the connectionrelation is changed so that a clock signal that is supplied to the gateelectrode of the seventh transistor 37 from the third input terminal 23and a clock signal that is supplied to the gate electrode of the eighthtransistor 38 from the second input terminal 22 are supplied from thesecond input terminal 22 and the third input terminal 23, respectively.In this case, in the shift register illustrated in FIG. 10A, the stateis changed from the state where both the seventh transistor 37 and theeighth transistor 38 are turned on, to the state where the seventhtransistor 37 is turned off and the eighth transistor 38 is turned on,and then to the state where both the seventh transistor 37 and theeighth transistor 38 are turned off; thus, the fall in an electricpotential of the node B due to fall in the electric potentials of thesecond input terminal 22 and the third input terminal 23 is caused twiceby fall in the electric potential of the gate electrode of the seventhtransistor 37 and fall in the electric potential of the gate electrodeof the eighth transistor 38. On the other hand, in the shift registerillustrated in FIG. 10A, the state is changed from the state where boththe seventh transistor 37 and the eighth transistor 38 are turned on tothe state where the seventh transistor 37 is turned on and the eighthtransistor 38 is turned off, and then to the state where both theseventh transistor 37 and the eighth transistor 38 are turned off.Accordingly, the fall in an electric potential of the node B due to fallin electric potentials of the second input terminal 22 and the thirdinput terminal 23 is reduced to one, which is caused by fall in anelectric potential of the gate electrode of the eighth transistor 38.Therefore, the connection relation, that is, the clock signal CK3 issupplied from the third input terminal 23 to the gate electrode of theseventh transistor 37 and the clock signal CK2 is supplied from thesecond input terminal 22 to the gate electrode of the eighth transistor38, is preferable. That is because the number of times of the change inthe electric potential of the node B can be reduced and the noise can bedecreased.

In this way, in a period during which the electric potential of thefirst output terminal 26 and the electric potential of the second outputterminal 27 are each held at an L level, an H level signal is regularlysupplied to the node B; therefore, malfunction of the pulse outputcircuit can be suppressed.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 4

The thin film transistor described in Embodiment 1 or 2 is manufactured,and a semiconductor device having a display function (also referred toas a display device) can be manufactured using the thin film transistorin a pixel portion and further in a driver circuit. Further, part or thewhole of the driver circuit having the thin film transistor described inEmbodiment 1 or 2 is formed over the same substrate as the pixelportion, whereby a system-on-panel can be obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. The light-emitting elementincludes, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically includes, in its category, aninorganic electroluminescent (EL) element, an organic EL element, andthe like. Furthermore, a display medium whose contrast is changed by anelectric effect, such as electronic ink, can be used.

In addition, the display device includes a panel in which a displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel. Furthermore, an element substrate,which corresponds to an embodiment before the display element iscompleted in a manufacturing process of the display device, is providedwith a means for supplying current to the display element in each of aplurality of pixels. The element substrate may be specifically in astate where only a pixel electrode of a display element is formed or ina state after a conductive film to be a pixel electrode is formed andbefore the conductive film is etched to form a pixel electrode, and canhave any mode.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device includes the following modules inits category: a module including a connector such as a flexible printedcircuit (FPC), a tape automated bonding (TAB) tape, or a tape carrierpackage (TCP) attached; a module having a TAB tape or a TCP which isprovided with a printed wiring board at the end thereof; and a modulehaving an integrated circuit (IC) which is directly mounted on a displayelement by a chip on glass (COG) method.

In this embodiment, the appearance and a cross section of a liquidcrystal display panel, which corresponds to one mode of a semiconductordevice, are described with reference to FIGS. 11A1, 11A2, and 11B. FIGS.11A1 and 11A2 are top views of a panel in which highly reliable thinfilm transistors 4010 and 4011 each including the In—Ga—Zn—O-based filmdescribed in Embodiments 1 and 2 as an oxide semiconductor layer and aliquid crystal element 4013 formed over a first substrate 4001 aresealed between the first substrate 4001 and a second substrate 4006 witha sealant 4005. FIG. 11B is a cross-sectional view taken along line M-Nof FIGS. 11A1 and 11A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Therefore, the pixelportion 4002 and the scan line driver circuit 4004 are sealed togetherwith a liquid crystal layer 4008, by the first substrate 4001, thesealant 4005, and the second substrate 4006. A signal line drivercircuit 4003 that is formed using a single crystal semiconductor film ora polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofa driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 11A1illustrates an example in which the signal line driver circuit 4003 ismounted by a COG method and FIG. 11A2 illustrates an example in whichthe signal line driver circuit 4003 is mounted by a TAB method.

Each of the pixel portion 4002 and the scan line driver circuit 4004which are provided over the first substrate 4001 includes a plurality ofthin film transistors. FIG. 11B illustrates the thin film transistor4010 included in the pixel portion 4002 and the thin film transistor4011 included in the scan line driver circuit 4004. Over the thin filmtransistors 4010 and 4011, insulating layers 4020 and 4021 are provided.

Any of the highly reliable thin film transistors including anIn—Ga—Zn—O-based film as the oxide semiconductor layers which aredescribed in Embodiments 1 and 2 can be used as the thin filmtransistors 4010 and 4011. In this embodiment, the thin film transistors4010 and 4011 are n-channel thin film transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is provided onthe second substrate 4006. A portion where the pixel electrode layer4030, the counter electrode layer 4031, and the liquid crystal layer4008 overlap with one another corresponds to the liquid crystal element4013. Note that the pixel electrode layer 4030 and the counter electrodelayer 4031 are provided with an insulating layer 4032 and an insulatinglayer 4033 which function as alignment films, respectively, and theliquid crystal layer 4008 is sandwiched between the pixel electrodelayer 4030 and the counter electrode layer 4031 with the insulatinglayers 4032 and 4033 interposed therebetween. Although not illustrated,a color filter may be provided on either the first substrate 4001 sideor the second substrate 4006 side.

Note that the first substrate 4001 and the second substrate 4006 can beformed of glass, metal (typically, stainless steel), ceramic, orplastic. As plastics, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. In addition, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films can beused.

A spacer 4035 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the distance (a cellgap) between the pixel electrode layer 4030 and the counter electrodelayer 4031. Alternatively, a spherical spacer may also be used. Inaddition, the counter electrode layer 4031 is electrically connected toa common potential line formed over the same substrate as the thin filmtransistor 4010. With the use of a common connection portion, thecounter electrode layer 4031 and the common potential line can beelectrically connected to each other by conductive particles arrangedbetween a pair of substrates. Note that the conductive particles areincluded in the sealant 4005.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which appears just before a cholesteric phase changesinto an isotropic phase while temperature of cholesteric liquid crystalis increased. Since the blue phase only appears within a narrow range oftemperature, a liquid crystal composition containing a chiral agent atgreater than or equal to 5 percent by weight so as to improve thetemperature range is used for the liquid crystal layer 4008. The liquidcrystal composition which includes a liquid crystal exhibiting a bluephase and a chiral agent has a short response time of greater than orequal to 10 pec and less than or equal to 100 pec, has optical isotropy,which makes the alignment process unneeded, and has a small viewingangle dependence.

Note that although an example of a transmissive liquid crystal displaydevice is described in this embodiment, the present invention can alsobe applied to a reflective liquid crystal display device or atransflective liquid crystal display device.

An example of the liquid crystal display device according to thisembodiment is described in which a polarizing plate is provided on theouter surface of the substrate (on the viewer side) and a coloring layerand an electrode layer used for a display element are provided on theinner surface of the substrate; however, the polarizing plate may beprovided on the inner surface of the substrate. The stacked structure ofthe polarizing plate and the coloring layer is not limited to thisembodiment and may be set as appropriate depending on materials of thepolarizing plate and the coloring layer or conditions of manufacturingprocess. Further, a light-blocking film functioning as a black matrixmay be provided.

In this embodiment, in order to reduce the surface roughness due to thethin film transistor and to improve the reliability of the thin filmtransistor, the thin film transistor obtained in Embodiment 1 or 2 iscovered with insulating layers (the insulating layers 4020 and 4021)functioning as a protective film and a planarizing insulating film. Notethat the protective film is provided to prevent entry of contaminantimpurities such as an organic substance, metal, and moisture existing inthe air and is preferably a dense film. The protective film may beformed to have a single-layer structure or a stacked-layer structureusing any of a silicon oxide film, a silicon nitride film, a siliconoxynitride film, a silicon nitride oxide film, an aluminum oxide film,an aluminum nitride film, an aluminum oxynitride film, and an aluminumnitride oxide film. Although this embodiment describes an example inwhich the protective film is formed by a sputtering method, any othermethod may be used.

In this embodiment, the insulating layer 4020 having a stacked-layerstructure is formed as the protective film. Here, a silicon oxide filmis formed by a sputtering method, as a first layer of the insulatinglayer 4020. The use of a silicon oxide film as a protective film has aneffect of preventing hillock of an aluminum film which is used as thesource and drain electrode layers.

As a second layer of the protective film, an insulating layer is formed.Here, a silicon nitride film is formed by a sputtering method, as asecond layer of the insulating layer 4020. The use of the siliconnitride film as the protective film can prevent mobile ions of sodium orthe like from entering a semiconductor region so that variation inelectrical characteristics of the TFT can be suppressed.

After the protective film is formed, annealing (higher than or equal to300° C. and lower than or equal to 400° C.) of the oxide semiconductorlayer may be performed.

The insulating layer 4021 is formed as the planarizing insulating film.The insulating layer 4021 may be formed using an organic material havingheat resistance such as acrylic, polyimide, benzocyclobutene, polyamide,or epoxy. Other than such organic materials, it is also possible to usea low-dielectric constant material (a low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. Note that the insulating layer 4021 may be formed by stackinga plurality of insulating films formed using these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include as a substituent anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.In addition, the organic group may include a fluoro group.

There is no particular limitation on the method of forming theinsulating layer 4021, and the following method or means can be employeddepending on the material: a method such as a sputtering method, an SOGmethod, a spin coating method, a dipping method, a spray coating method,or a droplet discharge method (e.g., an ink jet method, screen printing,or offset printing), or a tool such as a doctor knife, a roll coater, acurtain coater, or a knife coater. In the case of forming the insulatinglayer 4021 with the use of a liquid material, annealing (higher than orequal to 300° C. and lower than or equal to 400° C.) of the oxidesemiconductor layer may be performed at the same time as a baking step.The baking step of the insulating layer 4021 also serves as annealing ofthe oxide semiconductor layer, whereby a semiconductor device can bemanufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as anindium oxide containing a tungsten oxide, an indium zinc oxidecontaining a tungsten oxide, an indium oxide containing a titaniumoxide, an indium tin oxide containing a titanium oxide, an indium tinoxide (hereinafter referred to as ITO), an indium zinc oxide, an indiumtin oxide to which a silicon oxide is added, or the like.

Conductive compositions including a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed using the conductive composition preferably has a sheetresistance of less than or equal to 10000 ohms per square and a lighttransmittance of greater than or equal to 70% at a wavelength of 550 nm.Further, the resistivity of the conductive high molecule included in theconductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of two or more kinds of them, and thelike can be given.

Further, a variety of signals and electric potentials are supplied tothe signal line driver circuit 4003 which is formed separately, the scanline driver circuit 4004, or the pixel portion 4002 from an FPC 4018.

In this embodiment, a connection terminal electrode 4015 is formed usingthe same conductive film that is used for the pixel electrode layer 4030included in the liquid crystal element 4013. A terminal electrode 4016is formed using the same conductive film that is used for the source anddrain electrode layers of the thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 via an anisotropic conductive film4019.

FIGS. 11A1, 11A2, and 11B illustrate an example in which the signal linedriver circuit 4003 is formed separately and mounted on the firstsubstrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

FIG. 12 illustrates an example in which a liquid crystal display moduleis formed as a semiconductor device, using a TFT substrate 2600 which ismanufactured using the thin film transistor described in Embodiment 1 or2.

FIG. 12 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTor the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the RGB system, respective coloring layers corresponding tocolors of red, green, and blue are provided for respective pixels.Polarizing plates 2606 and 2607 and a diffusion plate 2613 are providedoutside the TFT substrate 2600 and the counter substrate 2601. A lightsource includes a cold cathode tube 2610 and a reflective plate 2611,and a circuit substrate 2612 is connected to a wiring circuit portion2608 of the TFT substrate 2600 by a flexible wiring board 2609 andincludes an external circuit such as a control circuit or a power supplycircuit. The polarizing plate and the liquid crystal layer may bestacked with a retardation plate interposed therebetween.

The liquid crystal display module can employ a twisted nematic (TN)mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS)mode, a multi-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, an axially symmetric aligned micro-cell (ASM)mode, an optically compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an anti ferroelectric liquid crystal (AFLC)mode, or the like.

Through the above process, a highly reliable liquid crystal displaypanel as a semiconductor device can be manufactured.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 5

In this embodiment, an example of electronic paper will be described asa semiconductor device to which the thin film transistor described inEmbodiment 1 or Embodiment 2 is applied.

FIG. 13 illustrates active matrix electronic paper as an example of asemiconductor device. As a thin film transistor 581 used for thesemiconductor device, the thin film transistor described in Embodiments1 and 2 can be applied.

The electronic paper in FIG. 13 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black and whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and anelectric potential difference is generated between the first electrodelayer and the second electrode layer to control orientation of thespherical particles, so that display is performed.

The thin film transistor 581 sealed between a substrate 580 and asubstrate 596 is a thin film transistor with a bottom-gate structure,and a source or drain electrode layer thereof is in contact with a firstelectrode layer 587 through an opening formed in insulating layers 584and 585, whereby the thin film transistor 581 is electrically connectedto the first electrode layer 587. Between the first electrode layer 587and a second electrode layer 588, spherical particles 589 are provided.Each spherical particle 589 includes a black region 590 a and a whiteregion 590 b, and a cavity 594 filled with liquid around the blackregion 590 a and the white region 590 b. The circumference of thespherical particle 589 is filled with a filler 595 such as a resin orthe like (see FIG. 13). In this embodiment, the first electrode layer587 corresponds to a pixel electrode, and the second electrode layer 588corresponds to a common electrode. The second electrode layer 588 iselectrically connected to a common potential line provided over the samesubstrate as the thin film transistor 581. With the use of the commonconnection portion described in Embodiment 1 or 2, the second electrodelayer 588 can be electrically connected to the common potential line byconductive particles arranged between a pair of substrates.

Further, instead of the twisting ball, an electrophoretic element canalso be used. A microcapsule having a diameter of approximately greaterthan or equal to 10 μm and less than or equal to 200 μm in whichtransparent liquid, positively charged white microparticles, andnegatively charged black microparticles are encapsulated, is used. Inthe microcapsule which is provided between the first electrode layer andthe second electrode layer, when an electric field is applied by thefirst electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides, sothat white or black can be displayed. A display element using thisprinciple is an electrophoretic display element and is generally calledelectronic paper. The electrophoretic display element has higherreflectance than a liquid crystal display element, and thus, anauxiliary light is unnecessary, power consumption is low, and a displayportion can be recognized in a dim place. In addition, even when poweris not supplied to the display portion, an image which has beendisplayed once can be maintained. Accordingly, a displayed image can bestored even if a semiconductor device having a display function (whichmay be referred to simply as a display device or a semiconductor deviceprovided with a display device) is distanced from an electric wavesource.

Through the above process, a highly reliable electronic paper as asemiconductor device can be realized.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 6

In this embodiment, an example of a light-emitting display device willbe described as a semiconductor device to which the thin film transistordescribed in Embodiment 1 or 2 is applied. As a display element includedin a display device, a light-emitting element utilizingelectroluminescence is described here. Light-emitting elements utilizingelectroluminescence are classified according to whether a light-emittingmaterial is an organic compound or an inorganic compound. In general,the former is referred to as an organic EL element, and the latter isreferred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Because of such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element as a light-emitting element is described here.

FIG. 14 illustrates an example of a pixel structure to which digitaltime grayscale driving can be applied, as an example of a semiconductordevice to which the present invention is applied.

A structure and operation of a pixel to which digital time grayscaledriving can be applied are described. Here, an example is described inwhich one pixel includes two n-channel transistors each of which isdescribed in Embodiments 1 and 2 and each of which includes an oxidesemiconductor layer (In—Ga—Zn—O-based film) in the channel formationregion.

A pixel 6400 includes a switching transistor 6401, a driver transistor6402, a light-emitting element 6404, and a capacitor 6403. A gate of theswitching transistor 6401 is connected to a scan line 6406, a firstelectrode (one of a source electrode and a drain electrode) of theswitching transistor 6401 is connected to a signal line 6405, and asecond electrode (the other of the source electrode and the drainelectrode) of the switching transistor 6401 is connected to a gate ofthe driver transistor 6402. The gate of the driver transistor 6402 isconnected to a power supply line 6407 via the capacitor 6403, a firstelectrode of the driver transistor 6402 is connected to the power supplyline 6407, and a second electrode of the driver transistor 6402 isconnected to a first electrode (pixel electrode) of the light-emittingelement 6404. A second electrode of the light-emitting element 6404corresponds to a common electrode 6408. The common electrode 6408 iselectrically connected to a common potential line provided over the samesubstrate. The connection portion may be used as a common connectionportion.

The second electrode (common electrode 6408) of the light-emittingelement 6404 is set to low power supply potential. Note that the lowpower supply potential is potential satisfying the low power supplypotential <high power supply potential with reference to the high powersupply potential that is set to the power supply line 6407. As the lowpower supply potential, a GND potential, 0 V, or the like may beemployed, for example. An electric potential difference between the highpower supply potential and the low power supply potential is applied tothe light-emitting element 6404 and current is supplied to thelight-emitting element 6404, so that the light-emitting element 6404emits light. Here, in order to make the light-emitting element 6404 emitlight, each electric potential is set so that the electric potentialdifference between the high power supply potential and the low powersupply potential is greater than or equal to forward threshold voltageof the light-emitting element 6404.

Note that a gate capacitor of the driver transistor 6402 may be used asa substitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitor of the driver transistor 6402 may be formedbetween the channel region and the gate electrode.

In the case of a voltage-input voltage driving method, a video signal isinput to the gate of the driver transistor 6402 so that the drivertransistor 6402 is in either of two states of being sufficiently turnedon or turned off. That is, the driver transistor 6402 operates in alinear region. Since the driver transistor 6402 operates in the linearregion, voltage greater than the voltage of the power supply line 6407is applied to the gate of the driver transistor 6402. Note that voltagegreater higher than or equal to (voltage of the power supply line+Vth ofthe driver transistor 6402) is applied to the signal line 6405.

In the case of performing analog grayscale driving instead of digitaltime grayscale driving, the same pixel configuration as FIG. 14 can beused by changing signal input.

In the case of performing analog grayscale driving, voltage greater thanor equal to (forward voltage of the light-emitting element 6404+Vth ofthe driver transistor 6402) is applied to the gate of the drivertransistor 6402. The forward voltage of the light-emitting element 6404indicates voltage at which a desired luminance is obtained, and includesat least forward threshold voltage. The video signal by which the drivertransistor 6402 operates in a saturation region is input, so thatcurrent can be supplied to the light-emitting element 6404. In order forthe driver transistor 6402 to operate in the saturation region, theelectric potential of the power supply line 6407 is set greater than thegate potential of the driver transistor 6402. When an analog videosignal is used, it is possible to feed current to the light-emittingelement 6404 in accordance with the video signal and perform analoggrayscale driving.

Note that the pixel structure is not limited to that illustrated in FIG.14. For example, a switch, a resistor, a capacitor, a transistor, alogic circuit, or the like may be added to the pixel illustrated in FIG.14.

Next, structures of the light-emitting element will be described withreference to FIGS. 15A to 15C. Here, the case where a driver TFT is ann-channel transistor is illustrated, and cross-sectional structures ofpixels are described. Driver TFTs 7001, 7011, and 7021 used forsemiconductor devices illustrated in FIGS. 15A to 15C can bemanufactured in a manner similar to that of the thin film transistordescribed in Embodiments 1 and 2 and are highly reliable thin filmtransistors each including an In—Ga—Zn—O-based film as an oxidesemiconductor layer.

In order to extract light emitted from the light-emitting element, atleast one of an anode and a cathode is required to transmit light. Athin film transistor and a light-emitting element are formed over asubstrate. A light-emitting element can have a top emission structure,in which light emission is extracted through the surface on the sideopposite to the substrate side; a bottom emission structure, in whichlight emission is extracted through the surface on the substrate side;or a dual emission structure, in which light emission is extractedthrough the surface on the side opposite to the substrate side and thesurface on the substrate side. The pixel structure according to oneembodiment of the present invention can be applied to a light-emittingelement having any of these emission structures.

Next, a light-emitting element having a bottom emission structure willbe described with reference to FIG. 15A.

FIG. 15A is a cross-sectional view of a pixel in the case where thedriver TFT 7011 is an n-channel transistor and light generated in alight-emitting element 7012 is emitted to pass through a first electrode7013. In FIG. 15A, the first electrode 7013 of the light-emittingelement 7012 is formed over a light-transmitting conductive film 7017which is electrically connected to the drain electrode layer of thedriver TFT 7011, and an EL layer 7014 and a second electrode 7015 arestacked in that order over the first electrode 7013.

As the light-transmitting conductive film 7017, a light-transmittingconductive film such as a film of an indium oxide containing a tungstenoxide, an indium zinc oxide containing a tungsten oxide, an indium oxidecontaining a titanium oxide, an indium tin oxide containing a titaniumoxide, an indium tin oxide, an indium zinc oxide, or an indium tin oxideto which a silicon oxide is added can be used.

Any of a variety of materials can be used for the first electrode 7013of the light-emitting element. For example, when the first electrode7013 serves as cathode, specifically, the first electrode 7013 ispreferably formed using a material having a low work function such as analkali metal such as Li or Cs; an alkaline earth metal such as Mg, Ca,or Sr; an alloy containing any of these metals (e.g., Mg:Ag or Al:Li);or a rare earth metal such as Yb or Er. In FIG. 15A, the first electrode7013 is formed to have a thickness enough to transmit light (preferably,approximately 5 nm to 30 nm). For example, an aluminum film with athickness of 20 nm is used as the first electrode 7013.

Alternatively, a light-transmitting conductive film and an aluminum filmmay be stacked and then selectively etched so as to form thelight-transmitting conductive film 7017 and the first electrode 7013. Inthis case, the etching can be performed using the same mask, which ispreferable.

The peripheral portion of the first electrode 7013 is covered with apartition 7019. The partition 7019 can be formed using an organic resinfilm of polyimide, acrylic, polyamide, epoxy, or the like; an inorganicinsulating film; or organic polysiloxane. It is particularly preferablethat the partition 7019 be formed using a photosensitive resin materialto have an opening over the first electrode 7013 so that a sidewall ofthe opening is formed as an inclined surface with continuous curvature.In the case where a photosensitive resin material is used for thepartition 7019, a step of forming a resist mask can be omitted.

The EL layer 7014 which is formed over the first electrode 7013 and thepartition 7019 may include at least a light-emitting layer and be formedusing a single layer or a plurality of layers stacked. When the EL layer7014 is formed using a plurality of layers, an electron-injection layer,an electron-transport layer, a light-emitting layer, a hole-transportlayer, and a hole-injection layer are stacked in that order over thefirst electrode 7013 which functions as a cathode. Note that it is notnecessary to form all of these layers.

The stacking order is not limited to the above stacking order, and ahole-injection layer, a hole-transport layer, a light-emitting layer, anelectron-transport layer, and an electron-injection layer may be stackedin that order over the first electrode 7013 which functions as an anode.However, when power consumption is compared, it is preferable that thefirst electrode 7013 function as a cathode and an electron-injectionlayer, an electron-transport layer, a light-emitting layer, ahole-transport layer, and a hole-injection layer be stacked in thatorder over the first electrode 7013 because voltage rise in the drivercircuit portion can be suppressed and power consumption can bedecreased.

As the second electrode 7015 formed over the EL layer 7014, variousmaterials can be used. For example, when the second electrode 7015 isused as an anode, it is preferable to use a material having a high workfunction, such as ZrN, Ti, W, Ni, Pt, Cr, or a light-transmittingconductive material such as ITO, IZO, or ZnO. Further, a light-blockingfilm 7016, for example, a metal which blocks light, a metal whichreflects light, or the like is provided over the second electrode 7015.In this embodiment, an ITO film is used as the second electrode 7015 anda Ti film is used as the light-blocking film 7016.

The light-emitting element 7012 corresponds to a region where the ELlayer 7014 including a light-emitting layer is sandwiched between thefirst electrode 7013 and the second electrode 7015. In the case of theelement structure illustrated in FIG. 15A, light is emitted from thelight-emitting element 7012 to the first electrode 7013 side asindicated by an arrow.

Note that in FIG. 15A, light emitted from the light-emitting element7012 passes through a color filter layer 7033, an insulating layer 7032,an oxide insulating layer 7031, a gate insulating layer 7060, and asubstrate 7010 to be emitted to the outside.

The color filter layer 7033 is formed by a droplet discharge method suchas an inkjet method, a printing method, an etching method with the useof a photolithography technique, or the like.

The color filter layer 7033 is covered with an overcoat layer 7034, andalso covered with a protective insulating layer 7035. Although theovercoat layer 7034 is illustrated to have a small thickness in FIG.15A, the overcoat layer 7034 has a function of reducing unevennesscaused by the color filter layer 7033 with the use of a resin materialsuch as an acrylic resin.

A contact hole which is formed in the protective insulating layer 7035and the insulating layer 7032, and which reaches a connection electrodelayer 7030 is provided in a portion which overlaps with the partition7019.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 15B.

In FIG. 15B, a first electrode 7023 of a light-emitting element 7022 isformed over a light-transmitting conductive film 7027 which iselectrically connected to the drain electrode layer of the driver TFT7021, and an EL layer 7024 and a second electrode 7025 are stacked inthat order over the first electrode 7023.

As the light-transmitting conductive film 7027, a light-transmittingconductive film such as a film of an indium oxide containing a tungstenoxide, an indium zinc oxide containing a tungsten oxide, an indium oxidecontaining a titanium oxide, an indium tin oxide containing a titaniumoxide, an indium tin oxide, an indium zinc oxide, or an indium tin oxideto which a silicon oxide is added can be used.

Any of a variety of materials can be used for the first electrode 7023.For example, when the first electrode 7023 serves as a cathode,specifically, the first electrode 7023 is preferably formed using amaterial having a low work function such as an alkali metal such as Lior Cs; an alkaline earth metal such as Mg, Ca, or Sr; an alloycontaining any of these metals (e.g., Mg:Ag or Al:Li); or a rare earthmetal such as Yb or Er. In this embodiment, the first electrode 7023serves as a cathode and the first electrode 7023 is formed to have athickness enough to transmit light (preferably, approximately 5 nm to 30nm). For example, an aluminum film with a thickness of 20 nm can be usedas the cathode.

Alternatively, a light-transmitting conductive film and an aluminum filmmay be stacked and then selectively etched so as to form thelight-transmitting conductive film 7027 and the first electrode 7023. Inthis case, the etching can be performed using the same mask, which ispreferable.

The peripheral portion of the first electrode 7023 is covered with apartition 7029. The partition 7029 can be formed using an organic resinfilm of polyimide, acrylic, polyamide, epoxy, or the like; an inorganicinsulating film; or organic polysiloxane. It is particularly preferablethat the partition 7029 be formed using a photosensitive material tohave an opening over the first electrode 7023 so that a sidewall of theopening is formed as an inclined surface with continuous curvature. Inthe case where a photosensitive resin material is used for the partition7029, a step of forming a resist mask can be omitted.

The EL layer 7024 which is formed over the first electrode 7023 and thepartition 7029 may include at least a light-emitting layer and be formedusing a single layer or a plurality of layers stacked. When the EL layer7024 is formed using a plurality of layers, an electron-injection layer,an electron-transport layer, a light-emitting layer, a hole-transportlayer, and a hole-injection layer are stacked in that order over thefirst electrode 7023 which functions as a cathode. Note that it is notnecessary to form all of these layers.

The stacking order is not limited to the above, and the first electrode7023 is used as an anode and a hole-injection layer, a hole-transportlayer, a light-emitting layer, an electron-transport layer, and anelectron-injection layer may be stacked in that order over the firstelectrode 7023. Note that when power consumption is compared, it ispreferable that the first electrode 7023 function as a cathode and anelectron-injection layer, an electron-transport layer, a light-emittinglayer, a hole-transport layer, and a hole-injection layer be stacked inthat order over the cathode because power consumption can be decreased.

As the second electrode 7025 formed over the EL layer 7024, variousmaterials can be used. For example, when the second electrode 7025 isused as an anode, it is preferable to use a material having a high workfunction, such as a light-transmitting conductive material such as ITO,IZO, or ZnO. In this embodiment, the second electrode 7025 is used as ananode, and an ITO film containing a silicon oxide is formed.

The light-emitting element 7022 corresponds to a region where the ELlayer 7024 including a light-emitting layer is sandwiched between thefirst electrode 7023 and the second electrode 7025. In the case of theelement structure illustrated in FIG. 15B, light emitted from thelight-emitting element 7022 is emitted from both the second electrode7025 side and the first electrode 7023 side as indicated by arrows.

Note that in FIG. 15B, light emitted from the light-emitting element7022 to the first electrode 7023 side passes through a color filterlayer 7043, an insulating layer 7042, an oxide insulating layer 7041, agate insulating layer 7070, and a substrate 7020 to be emitted to theoutside.

The color filter layer 7043 is formed by a droplet discharge method suchas an inkjet method, a printing method, an etching method with the useof a photolithography technique, or the like.

The color filter layer 7043 is covered with an overcoat layer 7044, andalso covered with a protective insulating layer 7045.

A contact hole which is formed in the protective insulating layer 7045and the insulating layer 7042 and which reaches a connection electrodelayer 7040 is provided in a portion which overlaps with the partition7029.

Note that in the case where the light-emitting element having a dualemission structure is used and full color display is performed on bothdisplay surfaces, light from the second electrode 7025 side does notpass through the color filter layer 7043; therefore, a sealing substrateprovided with another color filter layer is preferably provided on thesecond electrode 7025.

Next, a light-emitting element having a top emission structure will bedescribed with reference to FIG. 15C.

FIG. 15C is a cross-sectional view of a pixel in the case where thedriver TFT 7001 is an n-channel TFT and light generated in alight-emitting element 7002 is emitted to pass through a secondelectrode 7005. In FIG. 15C, a first electrode 7003 of thelight-emitting element 7002 is formed to be electrically connected tothe drain electrode layer of the driver TFT 7001, and an EL layer 7004and the second electrode 7005 are stacked in that order over the firstelectrode 7003.

The first electrode 7003 can be formed using any of a variety ofmaterials; for example, when the first electrode 7003 is used as acathode, it is preferable to use a material having a low work function,such as an alkali metal such as Li or Cs, an alkaline earth metal suchas Mg, Ca, or Sr, an alloy containing any of these metals (e.g., Mg:Ag,Al:Li), or a rare earth metal such as Yb or Er.

The EL layer 7004 which is formed over the first electrode 7003 and thepartition 7009 may include at least a light-emitting layer and be formedusing a single layer or a plurality of layers stacked. When the EL layer7004 is formed using a plurality of layers, the EL layer 7004 is formedby stacking an electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerin that order over the first electrode 7003. Note that it is notnecessary to form all of these layers.

The stacking order is not limited to the above stacking order, and ahole-injection layer, a hole-transport layer, a light-emitting layer, anelectron-transport layer, and an electron-injection layer may be stackedin that order over the first electrode 7003 which is used as an anode.

In FIG. 15C, a hole-injection layer, a hole-transport layer, alight-emitting layer, an electron-transport layer, and anelectron-injection layer are stacked in that order over a laminate filmin which a Ti film, an aluminum film, and a Ti film are stacked in thatorder. Further, a stacked layer of a Mg:Ag alloy thin film and an ITOfilm is formed.

Note that when the TFT 7001 is an n-channel transistor, it is preferablethat an electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerbe stacked in that order over the first electrode 7003 because anincrease in voltage of the driver circuit can be suppressed and powerconsumption can be decreased.

The second electrode 7005 is formed using a light-transmittingconductive material; for example, a light-transmitting conductive filmof an indium oxide containing a tungsten oxide, an indium zinc oxidecontaining a tungsten oxide, an indium oxide containing a titaniumoxide, an indium tin oxide containing a titanium oxide, an indium tinoxide, an indium zinc oxide, an indium tin oxide to which a siliconoxide is added, or the like can be used.

The light-emitting element 7002 corresponds to a region where the ELlayer 7004 including a light-emitting layer is sandwiched between thefirst electrode 7003 and the second electrode 7005. In the case of theelement structure illustrated in FIG. 15C, light is emitted from thelight-emitting element 7002 to the second electrode 7005 side asindicated by an arrow.

In FIG. 15C, the drain electrode layer of the TFT 7001 is electricallyconnected to the first electrode 7003 through a contact hole formed inan oxide insulating layer 7051, a protective insulating layer 7052, andan insulating layer 7055. A planarizing insulating layer 7053 can beformed using a resin material such as polyimide, acrylic,benzocyclobutene, polyamide, or epoxy. In addition to such resinmaterials, it is also possible to use a low-dielectric constant material(low-k material), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like. Note that the planarizinginsulating layer 7053 may be formed by stacking a plurality ofinsulating films formed of these materials. There is no particularlimitation on the method for forming the planarizing insulating layer7053, and the planarizing insulating layer 7053 can be formed, dependingon the material, by a method such as a sputtering method, an SOG method,spin coating, dipping, spray coating, or a droplet discharge method(e.g., an ink jet method, screen printing, or offset printing), or witha tool (equipment) such as a doctor knife, a roll coater, a curtaincoater, or a knife coater.

The partition 7009 is provided so as to insulate the first electrode7003 and the first electrode 7003 of an adjacent pixel. The partition7009 can be formed using an organic resin film of polyimide, acrylic,polyamide, epoxy, or the like; an inorganic insulating film; or organicpolysiloxane. It is particularly preferable that the partition 7009 beformed using a photosensitive resin material to have an opening over thefirst electrode 7003 so that a sidewall of the opening is formed as aninclined surface with continuous curvature. In the case where aphotosensitive resin material is used for the partition 7009, a step offorming a resist mask can be omitted.

In the structure illustrated in FIG. 15C, for performing full colordisplay, the light-emitting element 7002, one of light-emitting elementsadjacent to the light-emitting element 7002, and the other of thelight-emitting elements are, for example, a green emissivelight-emitting element, a red emissive light-emitting element, and ablue emissive light-emitting element, respectively. Alternatively, alight-emitting display device capable of full color display may bemanufactured using four kinds of light-emitting elements which include awhite emissive light-emitting element in addition to three kinds oflight-emitting elements.

In the structure of FIG. 15C, a light-emitting display device capable offull color display may be manufactured in such a way that all of aplurality of light-emitting elements which is arranged is white emissivelight-emitting elements and a sealing substrate having a color filter orthe like is placed on the light-emitting element 7002. A material whichexhibits a single color such as white is formed and combined with acolor filter or a color conversion layer, whereby full color display canbe performed.

Needless to say, display of monochromatic light can also be performed.For example, a lighting device may be formed with the use of white lightemission, or an area-color light-emitting device may be formed with theuse of monochromatic light emission.

If necessary, an optical film such as a polarizing film including acircularly polarizing plate may be provided.

Note that, although the organic EL elements are described here as thelight-emitting elements, an inorganic EL element can also be provided asa light-emitting element.

Note that the example is described in which a thin film transistor (adriver TFT) which controls the driving of a light-emitting element iselectrically connected to the light-emitting element; however, astructure may be employed in which a TFT for current control isconnected between the driver TFT and the light-emitting element.

The structure of the semiconductor device described in this embodimentis not limited to those illustrated in FIGS. 15A to 15C and can bemodified in various ways based on the spirit of techniques of thepresent invention.

Next, the appearance and the cross section of a light-emitting displaypanel (also referred to as a light-emitting panel) which corresponds toone embodiment of a semiconductor device to which the thin filmtransistor described in Embodiment 1 or 2 is applied are described withreference to FIGS. 16A and 16B. FIG. 16A is a top view of a panel inwhich thin film transistors and a light-emitting element formed over afirst substrate are sealed between the first substrate and a secondsubstrate with a sealant. FIG. 16B is a cross-sectional view taken alongline H-I of FIG. 16A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. It is preferable that a panel be packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air, in this manner.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors,and a thin film transistor 4510 included in the pixel portion 4502 and athin film transistor 4509 included in the signal line driver circuit4503 a are illustrated as an example in FIG. 16B.

Any of the highly reliable thin film transistors including anIn—Ga—Zn—O-based film as the oxide semiconductor layer which aredescribed in Embodiments 1 and 2 can be used as the thin filmtransistors 4509 and 4510. In this embodiment, the thin film transistors4509 and 4510 are n-channel thin film transistors.

Over an insulating layer 4544, a conductive layer 4540 is provided in aposition overlapping with a channel formation region of an oxidesemiconductor layer of the thin film transistor 4509 used for a drivercircuit. By providing the conductive layer 4540 so as to overlap withthe channel formation region of the oxide semiconductor layer, theamount of change in the threshold voltage of the thin film transistor4509 between before and after the BT test can be reduced. Further, theelectric potential of the conductive layer 4540 may be equal to ordifferent from that of a gate electrode layer of the thin filmtransistor 4509. The conductive layer 4540 can function also as a secondgate electrode layer. Alternatively, the electric potential of theconductive layer 4540 may be a GND potential or 0 V, or the conductivelayer 4540 may be in a floating state.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a source ordrain electrode layer of the thin film transistor 4510. Note that astructure of the light-emitting element 4511 is a stacked-layerstructure of the first electrode layer 4517, an electroluminescent layer4512, and a second electrode layer 4513, but there is no particularlimitation on the structure. The structure of the light-emitting element4511 can be changed as appropriate depending on the direction in whichlight is extracted from the light-emitting element 4511, or the like.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, or organic polysiloxane. It is particularly preferablethat the partition 4520 be formed using a photosensitive material and anopening be formed over the first electrode layer 4517 so that a sidewallof the opening is formed as an inclined surface with continuouscurvature.

The electroluminescent layer 4512 may be formed with a single layer or aplurality of layers stacked.

A protective film may be formed over the second electrode layer 4513 andthe partition 4520 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4511. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

In addition, a variety of signals and electric potentials are suppliedto the signal line driver circuits 4503 a and 4503 b, the scan linedriver circuits 4504 a and 4504 b, or the pixel portion 4502 from FPCs4518 a and 4518 b.

In this embodiment, a connection terminal electrode 4515 is formed usingthe same conductive film that is used for the first electrode layer 4517included in the light-emitting element 4511. A terminal electrode 4516is formed using the same conductive film that is used for the source anddrain electrode layers included in the thin film transistors 4509 and4510.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a via an anisotropic conductive film4519.

The second substrate located in the direction in which light isextracted from the light-emitting element 4511 should have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used for the second substrate.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, polyvinyl chloride (PVC), acrylic, polyimide, anepoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylenevinyl acetate (EVA) can be used. In this embodiment, nitrogen is usedfor the filler.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, an anti-glare treatment by which reflected light canbe diffused by projections and depressions on the surface so as toreduce the glare can be performed.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be mounted as driver circuitsformed using a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared. In addition,only the signal line driver circuits or part thereof, or the scan linedriver circuits or part thereof may be separately formed and mounted.This embodiment is not limited to the structure illustrated in FIGS. 16Aand 16B.

Through the above process, a highly reliable light-emitting displaydevice (display panel) as a semiconductor device can be manufactured.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 7

A semiconductor device to which the thin film transistor described inEmbodiment 1 or 2 is applied can be used as electronic paper. Electronicpaper can be used for electronic devices of a variety of fields as longas they can display data. For example, electronic paper can be appliedto an e-book reader (electronic book), a poster, an advertisement in avehicle such as a train, or displays of various cards such as a creditcard. Examples of the electronic devices are illustrated in FIGS. 17Aand 17B and FIG. 18.

FIG. 17A illustrates a poster 2631 using electronic paper. In the casewhere an advertising medium is printed paper, the advertisement isreplaced by hands; however, by using the electronic paper, theadvertising display can be changed in a short time. Furthermore, stableimages can be obtained without display defects. Note that the poster mayhave a configuration capable of wirelessly transmitting and receivingdata.

FIG. 17B illustrates an advertisement 2632 in a vehicle such as a train.In the case where an advertising medium is paper, the advertisement isreplaced by hand, but in the case where it is electronic paper, muchmanpower is not needed and the advertising display can be changed in ashort time. Furthermore, stable images can be obtained without displaydefects. Note that the advertisement in a vehicle may have aconfiguration capable of wirelessly transmitting and receiving data.

FIG. 18 illustrates an example of an e-book reader. For example, ane-book reader 2700 includes two housings, a housing 2701 and a housing2703. The housing 2701 and the housing 2703 are combined with a hinge2711 so that the e-book reader 2700 can be opened and closed with thehinge 2711 as an axis. With such a structure, the e-book reader 2700 canoperate like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the structure where different images are displayed indifferent display portions, for example, the right display portion (thedisplay portion 2705 in FIG. 18) can display text and the left displayportion (the display portion 2707 in FIG. 18) can display an image.

In the example illustrated in FIG. 18, the housing 2701 is provided withan operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, and the like may be provided onthe same surface as the display portion of the housing. Furthermore, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal that can be connected to various cables such as an AC adapterand a USB cable, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the e-book reader 2700 may have a function of anelectronic dictionary.

The e-book reader 2700 may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 8

A semiconductor device using the thin film transistor described inEmbodiment 1 or 2 can be applied to a variety of electronic appliances(including a game machine). Examples of electronic devices are atelevision device (also referred to as a television or a televisionreceiver), a monitor of a computer or the like, a camera such as adigital camera or a digital video camera, a digital photo frame, amobile phone (also referred to as a mobile phone handset or a mobilephone device), a portable game console, a portable information terminal,an audio reproducing device, a large-sized game machine such as apachinko machine, and the like.

FIG. 19A illustrates an example of a television device. In a televisiondevice 9600, a display portion 9603 is incorporated in a housing 9601.The display portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605.

The television device 9600 can be operated with an operation switch ofthe housing 9601 or a separate remote controller 9610. Channels andvolume can be controlled with an operation key 9609 of the remotecontroller 9610 so that an image displayed on the display portion 9603can be controlled. Furthermore, the remote controller 9610 may beprovided with a display portion 9607 for displaying data output from theremote controller 9610.

Note that the television device 9600 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device 9600is connected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 19B illustrates an example of a digital photo frame. For example,in a digital photo frame 9700, a display portion 9703 is incorporated ina housing 9701. The display portion 9703 can display a variety ofimages. For example, the display portion 9703 can display data of animage taken with a digital camera or the like and function as a normalphoto frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection portion (a USB terminal, a terminal thatcan be connected to various cables such as a USB cable, or the like), arecording medium insertion portion, and the like. Although thesecomponents may be provided on the surface on which the display portionis provided, it is preferable to provide them on the side surface or theback surface for the design of the digital photo frame 9700. Forexample, a memory storing data of an image taken with a digital camerais inserted in the recording medium insertion portion of the digitalphoto frame, whereby the image data can be transferred and thendisplayed on the display portion 9703.

The digital photo frame 9700 may be configured to transmit and receivedata wirelessly. The structure may be employed in which desired imagedata is transferred wirelessly to be displayed.

FIG. 20A is a portable game machine and is constituted by two housingsof a housing 9881 and a housing 9891 which are connected with a jointportion 9893 so that the portable game machine can be opened or folded.A display portion 9882 and a display portion 9883 are incorporated inthe housing 9881 and the housing 9891, respectively. In addition, theportable game machine illustrated in FIG. 20A is provided with a speakerportion 9884, a recording medium insertion portion 9886, an LED lamp9890, input means (operation keys 9885, a connection terminal 9887, asensor 9888 (having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotation number,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radial ray, flow rate, humidity, gradient, vibration, odor, or infraredray), and a microphone 9889), and the like. It is needless to say thatthe structure of the portable game machine is not limited to the aboveand other structures provided with at least a semiconductor device ofthe present invention may be employed. The portable game machine mayinclude other accessories, as appropriate. The portable game machineillustrated in FIG. 20A has a function of reading a program or datastored in the recording medium to display it on the display portion, anda function of sharing information with another portable game machine bywireless communication. Note that a function of the portable gamemachine illustrated in FIG. 20A is not limited to the above, and theportable game machine can have a variety of functions.

FIG. 20B illustrates an example of a slot machine which is a large-sizedgame machine. In a slot machine 9900, a display portion 9903 isincorporated in a housing 9901. In addition, the slot machine 9900includes an operation means such as a start lever or a stop switch, acoin slot, a speaker, and the like. It is needless to say that thestructure of the slot machine 9900 is not limited to the above and otherstructures provided with at least a semiconductor device of the presentinvention may be employed. The slot machine 9900 may include otheraccessories, as appropriate.

FIG. 21A illustrates an example of a mobile phone. A mobile phone 1000includes a display portion 1002 incorporated in a housing 1001, anoperation button 1003, an external connection port 1004, a speaker 1005,a microphone 1006, and the like.

When the display portion 1002 illustrated in FIG. 21A is touched with afinger or the like, data can be input into the mobile phone 1000.Furthermore, operations such as making calls and composing mails can beperformed by touching the display portion 1002 with a finger or thelike.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing a mail, a textinput mode mainly for inputting text is selected for the display portion1002 so that text displayed on a screen can be input. In that case, itis preferable to display a keyboard or number buttons on almost all areaof the screen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 1000, display in the screen of the display portion 1002 canbe automatically switched by determining the installation direction ofthe mobile phone 1000 (whether the mobile phone 1000 is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 1002 oroperating the operation button 1003 of the housing 1001. Alternatively,the screen modes may be switched depending on the kind of the imagedisplayed on the display portion 1002. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 1002 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken when thedisplay portion 1002 is touched with a palm or a finger, wherebypersonal identification can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

FIG. 21B also illustrates an example of a mobile phone. The mobile phonein FIG. 21B includes an a display device 9410 in which a display portion9412 and an operation button 9413 are included in a housing 9411, and acommunication device 9400 in which operation buttons 9402, an externalinput terminal 9403, a microphone 9404, a speaker 9405, and alight-emitting portion 9406 that emits light when a phone call isreceived are included in a housing 9401. The display device 9410 havinga display function can be detached from or attached to the communicationdevice 9400 having a telephone function in two directions as indicatedby arrows. Thus, a short axis of the display device 9410 can be attachedto a short axis of the communication device 9400, and a long axis of thedisplay device 9410 can be attached to a long axis of the communicationdevice 9400. In addition, when only the display function is needed, thedisplay device 9410 can be detached from the communication device 9400and used alone. Images or input information can be transmitted orreceived by wireless or wire communication between the communicationdevice 9400 and the display device 9410, each of which has arechargeable battery.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 9

In this embodiment, differences in a phenomenon in which oxygen moveswhen an oxide semiconductor layer is in contact with a metal layer (aconductive layer) or an oxide insulating layer between the case of anamorphous oxide semiconductor layer and the case of a crystalline oxidesemiconductor layer are described using scientific computation results.

FIG. 24 is a schematic view of a state where an oxide semiconductorlayer is in contact with a metal layer to serve as a source electrodelayer and a drain electrode layer and the oxide insulating layer in astructure of a thin film transistor which is one embodiment of thepresent invention. The directions of arrows indicate a direction ofmovement of oxygen in a state where these are in contact with each otheror a state where these are heated.

When oxygen vacancies occur, an i-type oxide semiconductor layer hasn-type conductivity, whereas when oxygen is oversupplied, an n-typeoxide semiconductor layer caused by oxygen vacancies becomes an i-typeoxide semiconductor layer. This effect is utilized in an actual deviceprocess, and in the oxide semiconductor layer which is in contact withthe metal layer to serve as a source electrode layer and a drainelectrode layer, oxygen is pulled to the metal side, and oxygenvacancies occur in part of a region, which is in contact with the metallayer (in the case of a small thickness, in an entire region in the filmthickness direction), whereby the oxide semiconductor layer becomes ann-type oxide semiconductor layer and favorable contact with the metallayer can be obtained. In addition, oxygen is supplied from the oxideinsulating layer to the oxide semiconductor layer in contact with theoxide insulating layer, and part of a region of the oxide semiconductorlayer, which is in contact with the oxide insulating layer (in the caseof a small thickness, in the entire region in the film thicknessdirection), contains excessive oxygen, to be an i-type region, wherebythe oxide semiconductor layer becomes an i-type oxide semiconductorlayer and functions as a channel formation region of a thin filmtransistor.

In one embodiment of the present invention, in a region where the oxidesemiconductor layer is in contact with the metal layer which serves as asource electrode layer and a drain electrode layer and the oxideinsulating layer, a crystal region is formed, and differences in oxygenmovement states between the case where the region is in an amorphousstate and the case where the region is a crystal region were examined byscientific computing.

Models used for scientific computing have an In—Ga—Zn—O-based amorphousstructure and an In—Ga—Zn—O-based crystal structure. In each of themodels, one of regions in a longitudinal direction of a rectangularsolid was deficient in oxygen by 10% as compared with the other region(see FIGS. 25A and 25B). The calculation is to compare distribution ofoxygen in the In—Ga—Zn—O-based amorphous structure and theIn—Ga—Zn—O-based crystal structure after ten nanoseconds under anaccelerated condition of 650° C. Respective conditions are shown inTable 1 and Table 2.

TABLE 1 Structural Condition Number of atoms 317(oxgen = 192) LatticeConstant a = b = 1.3196 nm, c = 2.6101 nm, α = β = 90°, γ = 120° Density6.23 g/cm³

TABLE 2 Calculation Contents Ensemble NTV(Number of atoms, Temperature,Volume fixing) Temperature 923 K Step size of time 0.2 fs Totalcalculate 10 ns time Potential Applying “Born-Mayer-Huggins type” to“Metal- Oxgen & Oxygen-Oxygen” Charge In: +3, Ga: +3, Zn: +2, O: −2

Distribution of oxygen in the case of using an amorphous oxidesemiconductor layer is shown in FIG. 26A, and distribution of oxygen inthe case of using a crystalline oxide semiconductor layer is shown inFIG. 26B. A dotted line indicates an initial state (Initial), and asolid line indicates a result (after ten nanoseconds). It is found thatoxygen moves regardless of whether the amorphous oxide semiconductorlayer or the crystalline oxide semiconductor layer is used.

The increasing rates of oxygen atoms between before and aftercalculation in a region having oxygen vacancies were 15.9% in the caseof the amorphous oxide semiconductor layer and 11.3% in the case of thecrystalline oxide semiconductor layer. That is, oxygen in the amorphousoxide semiconductor layer is more likely to move than oxygen in thecrystalline oxide semiconductor layer, resulting in easily compensatingfor the oxygen vacancies. In other words, oxygen in the crystallineoxide semiconductor layer is relatively less likely to move than oxygenin the amorphous oxide semiconductor layer.

Therefore, it is also found that oxygen moves in the oxide semiconductorlayer in one embodiment of the present invention having the crystalregion, in a manner similar to that of the case of the amorphous oxidesemiconductor layer. It is also found that the crystal region has aneffect in which elimination of oxygen from the oxide semiconductor layeris suppressed because oxygen is relatively less likely to move in thecrystalline oxide semiconductor layer than in the amorphous oxidesemiconductor layer.

This application is based on Japanese Patent Application serial no.2009-234413 filed with Japan Patent Office on Oct. 8, 2009, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a gate electrode layer; a gateinsulating layer over the gate electrode layer; an oxide semiconductorlayer over the gate insulating layer; a first oxide insulating layer incontact with part of the oxide semiconductor layer; and a sourceelectrode layer and a drain electrode layer in contact with part of theoxide semiconductor layer, wherein a region of the oxide semiconductorlayer located directly underneath a gap between the source electrode andthe first oxide insulating layer, and a region of the oxidesemiconductor layer located directly underneath a gap between the drainelectrode and the first oxide insulating layer, each has a thicknessless than a region overlapping with the source electrode layer, a regionoverlapping with the first oxide insulating layer, and a regionoverlapping with the drain electrode layer.